refactor: merge filterer and searcher concepts

[SychO9]

Part of #3884

Changes proposed in this pull request:

• Completely removes the concept of gambits from the backend.
• Merges the filterer and searcher systems into a single searcher system.
• Shared Query namespace is dropped.
• Searcher now have Filters instead of Gambits (and Fulltext Filters).
• Forum query parameters: it used to be when you search, the URL is changed to the following ?q=lorem is:locked. Since this doesn't work when the backend has to process that (not aware of gambits anymore). The URL is applied with filters auto transformed) into ?q=lorem&filter[locked]=true. We now also translates a set of filters into gambits (to fill the search input).
• Extensions such as subscriptions no longer has to append to q to filter the discussion list, it only adds an actual filter.

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.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

[SychO9]

I'll merge any and all search-related PRs into a separate #3893 branch so that they can still be reviewed before merging this new branch into 2.x (I need to continue building on top of all the work done so far).