[imorland]

Requested on Discuss: https://discuss.flarum.org/d/40038-introducing-deck-follow-every-conversation-you-care-about-live/4 (no issue)

Changes proposed in this pull request:
Adds a group: filter for discussions: group:mods lists the discussions started by a member of Mods. Groups can be named by ID, singular or plural name in any case, a comma list matches any of them, and -group: excludes them. The existing GroupGambit is now registered for discussions too, so the forum search and Deck's custom filter columns accept it, e.g. group:mods is:unread -author:me.

Until now group: only existed for users; in a discussion search it fell through to a text search for the words "group" and the group's name.

Reviewers should focus on:

• Groups the actor can't see match nothing, so a hidden group's members aren't revealed. group:members matches every registered author, as Deck's posts authorGroup filter does; -group: includes discussions whose author was deleted.
• Names are compared in PHP rather than with SQL lower(), which only folds ASCII on SQLite.
• GlobalSearchState no longer writes a filter twice now that discussions and users share a gambit (filter[group]=mods became group:mods group:mods).

Screenshot

The search window's suggestions now offer group: for discussions.

Before	After

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.

Required changes:

• Related documentation PR: docs(deck): mention the group: filter for discussions docs#709