[1.x] [extensibility] feat: allow DiscussionsSearchSource to be extended

[imorland]

Changes proposed in this pull request:
This splits DiscussionsSearchSource out so that it is possible to extend/customize the following:

On DiscussionsSearchSource

• includes - allow adding additional includes from the frontend.
• limit - allow setting a different results limit
• queryMutators - allow adding additional gambits to the default query. ie, from fof/best-answer, we can now choose to exclude best answer results from the normal discussions results, as this extension adds it's own search source with best answer results.

New DiscussionsSearchItem
This handles how the elements are presented within the search dropdown. We can customize:

• discussionTitle - ie, for within a translation extension so that titles can be presented already translated
• mostRelevantPostContent - as above
• viewItems - add/remove/modify elements of the view by dealing with an ItemList

Reviewers should focus on:

Screenshot

QA

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.