Implement more gambits

[tobyzerner]

Gambits are little pieces of search criteria that can be specified in the search string. Just like GitHub's search syntax.

Right now we've only implemented a few: is:unread and author:username in core, and tag:foo,bar, is:followed, is:ignored, is:sticky, and is:locked in their respective extensions.

We probably want to implement the following gambits:

• Multiple author search, separated by commas (author:username1,username2)
• created:date where date is the same syntax as on GitHub
• updated:date
• contributor:username1,username2
• replies:number where number is the same syntax as on GitHub
• ip:123.123.123.123

Pending flarum/framework#257, we will also want to have an in:title gambit which scopes fulltext search to only look in discussion titles.

We should also implement more gambits when searching users (i.e. /api/users?q=whatever), such as for filtering by group.

This is a precursor to an extensible advanced search interface which will come later.