WIP: Split Search Filter and Search

[askvortsov1]

Start of flarum/issue-archive#286

Changes proposed in this pull request:

• Split search and filter into separate endpoints
• Add a "filterer" that applies filters and filter mutations

TODO:

• Add filter equivalent of default search gambits
• Convert bundled extensions to use this system
• Clean up code / improve abstractions
• Convert ListPostsController to use this system (in a separate PR)

Reviewers should focus on:

• If we register things both as filters and as gambits for SimpleFlarumSearch, we're gonna have a lot of duplicated code. That being said, I'm not sure if there's anything we can do here if we want to truly separate the two
• For simplicity and to reduce diff size, I omitted some of the abstractions present in the old search system for filtering (SearchCriteria, etc). I also reused some SimpleFlarumSearch utils / components, like SearchResults, ApplySearchParametersTrait, AbstractSearch. What do we want to do about this? IMO simpler is better, especially seeing that custom filtration drivers aren't going to be a thing, so unnecessary abstraction is unnecessary. Did I cut out too much? Also, I want to eventually move the shared stuff into a Query namespace, but didn't do so here to minimize diff.

What if we used one endpoint, and searched vs filtered based on the presence of the q parameter? And what if SimpleFlarumSearch internally used filtration as its gambits (and all gambits were applied as key-values on the filter param instead of as part of the q string?

[askvortsov1]

Superceded by #2454