[imorland]

Related to flarum/framework#5178 (Filter columns don't follow changes that move a discussion in or out of their filters)

Changes proposed in this pull request:

Deck's filter columns now follow changes that move a discussion in or out of them without a reply, such as locking, stickying and re-tagging. This documents that for admins on the Deck page, and for developers on the extending page: the new 'recheck' realtime result, the optional recheck() method on sources, and new DeckColumns().membershipEvents(...) for extensions that broadcast their own such events.

Reviewers should focus on:

• Whether the best-answer example reads clearly for extension authors.

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.