Added in:<channels> search predicate

[talneoran]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

See #2299 for more information.
This PR adds a search predicate (according to #1237) that allows filtering messages based on the channel they were sent in, meant for usage in /mentions.
This is also inspired by the similar filter in Discord.
Example usage:
in:fourtf would show messages originally sent in fourtf channel.
in:fourtf,pajlada would show messages originally sent in either fourtf or pajlada channels.

The things I'm not completely sure about now are:

1. Currently the filter works everywhere, but is only useful in /mentions. If used in a regular split it will either show all messages or none, depending on whether or not the channel list includes the current channel. I'm wondering if there is a need to enforce this filter only being useable in /mentions, or if the current state is fine (at least to me the current functionality makes sense).
2. The wording of the changelog entry / explanation of the feature.

Closes #2299

[leon-richardt]

Tested for a few simple scenarios, and works as expected. 👍 I think this is a great use case for message search predicates (although I am probably a little biased 🙂)

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I'm wondering if there is a need to enforce this filter only being useable in /mentions, or if the current state is fine (at least to me the current functionality makes sense).

I think the current state is fine. The behavior makes sense, and restricting it to /mentions now might only limit future use cases

[pajlada]

LGTM 👍 Will update branch and merge in after #2633