Extended search field

[iPurpl3x]

Changes proposed in this pull request:

Breaking:

• The filter dropdown now does inner joins instead of outer joins when multiple groups are selected

New:

• The Search Filed can now be used to filter users by selecting groups and more
• This feature is extensible. Other extensions can register search types, by extending from forum/searchType/AbstractType.

Credits of for this change go mainly to @clarkwinkelmann who initially added this extended search filed to this extension one year ago! (see 9f3ed08)

Minor Changes:

• Optimized logic of filter dropdown
• The filter dropdown now changes the URL

Reviewers should focus on:
This is purely a front-end enhancement. Just check that the new feature works and the old filter dropdown isn't broken.

Screenshot

(Screenshot is from https://community.skillsnet.swiss, who is sponsoring this feature!)

Confirmed

• Frontend changes: tested on a local Flarum installation.