Rename "channels" to "conversations."

The term "channel" for a conversational container has a long and storied history, but is mostly evocative of IRC and of other, ah, "nerd-centric" services. It does show up in more widespread contexts: Discord and Slack both refer to their primary conversational containers as "channels," for example. However, I think it's unnecessary jargon, and I'd like to do away with it.

To that end, this change pervasively changes one term to the other wherever it appears, with the following exceptions:

* A `channel` concept (unrelated to conversations) is also provided by an external library; we can't and shouldn't try to rename that.
* The code to deal with the `pilcrow:channelData` and `pilcrow:lastActiveChannel` local storage properties is still present, to migrate existing data to new keys. It will be removed in a later change.

This is a **breaking API change**. As we are not yet managing any API compatibility promises, this is formally not an issue, but it is something to be aware of practically. The major API changes are:

* Paths beginning with `/api/channels` are now under `/api/conversations`, without other modifications.
* Fields labelled with `channel…` terms are now labelled with `conversation…` terms. For example, a `message` `sent` event is now sent to a `conversation`, not a `channel`.

This is also a **breaking UI change**. Specifically, any saved paths for `/ch/CHANNELID` will now lead to a 404. The corresponding paths are `/c/CONVERSATIONID`. While I've made an effort to migrate the location of stored data, I have not tried to provide adapters to fix this specific issue, because the disruption is short-lived and very easily addressed by opening a channel in the client UI.

This change is obnoxiously large and difficult to review, for which I apologize. If this shows up in `git annotate`, please forgive me. These kinds of renamings are hard to carry out without a major disruption, especially when the concept ("channel" in this case) is used so pervasively throughout the system.

I think it's worth making this change that pervasively so that we don't have an indefinitely-long tail of "well, it's a conversation in the docs, but the table is called `channel` for historical reasons" type issues.

Merges conversations-not-channels into main.

2 files changed, 2 insertions, 2 deletions

diff --git a/docs/developer/server/code-organization.md b/docs/developer/server/code-organization.md
index d17b604..3a691a2 100644
--- a/docs/developer/server/code-organization.md
+++ b/docs/developer/server/code-organization.md
@@ -6,7 +6,7 @@ Trust your gut, and reorganize to meet new needs. We've already revised this sch
 
 ## Topic modules
 
-High-level concerns are grouped into topical modules. These include `crate::channel`, `crate::events`, `crate::login`, and others. Those modules generally contain their own app types, their own repo types, their own extractors, and any other supporting code they need. They may also provide an interface to other modules in the program.
+High-level concerns are grouped into topical modules. These include `crate::conversation`, `crate::events`, `crate::login`, and others. Those modules generally contain their own app types, their own repo types, their own extractors, and any other supporting code they need. They may also provide an interface to other modules in the program.
 
 Most topic modules contain one or more of:
 
diff --git a/docs/developer/server/testing.md b/docs/developer/server/testing.md
index 8e87568..a3109cb 100644
--- a/docs/developer/server/testing.md
+++ b/docs/developer/server/testing.md
@@ -22,6 +22,6 @@ Prefer writing "flat" fixtures that do one thing, over compound fixtures that do
 
 Prefer role-specific names for test values: use, for example, `sender` for a login related to sending messages, rather than `login`. Fixture data is cheap, so make as many entities as make sense for the test. They'll vanish at the end of the test anyways.
 
-Prefer testing a single endpoint at a time. Other interactions, which may be needed to set up the scenario or verify the results, should be done against the `app` abstraction directly. It's okay if this leads to redundant tests (see for example `src/channel/routes/test/on_send.rs` and `src/events/routes/test.rs`, which overlap heavily).
+Prefer testing a single endpoint at a time. Other interactions, which may be needed to set up the scenario or verify the results, should be done against the `app` abstraction directly. It's okay if this leads to redundant tests (see for example `src/conversation/routes/test/on_send.rs` and `src/events/routes/test.rs`, which overlap heavily).
 
 Panicking in tests is fine. Panic messages should describe why the preconditions were expected, and can be terse.