Improve search performance (#1339)

* Improve fulltext gambit

* Only search in visible posts

This change relies on the `visibility-scoping` branch to be merged.

* Change posts table to use InnoDB engine

Doing a JOIN between an InnoDB table (discussions) and a MyISAM table
(posts) is very very (very) bad for performance. FULLTEXT indexes are
fully supported in InnoDB now, and it is a superior engine in every
other way, so there is no longer any reason to be using MyISAM.

* Use ::class

* Only search for comment posts

* Add fulltext index to discussions.title

* Fix migration not working if there is a table prefix

* Update frontend appearance

* Apply fixes from StyleCI

[ci skip] [skip ci]

* Show search result excerpts on mobile

js/lib/models/Discussion.js

@@ -21,7 +21,7 @@ Object.assign(Discussion.prototype, {
   commentsCount: Model.attribute('commentsCount'),
   repliesCount: computed('commentsCount', commentsCount => Math.max(0, commentsCount - 1)),
   posts: Model.hasMany('posts'),
-  relevantPosts: Model.hasMany('relevantPosts'),
+  mostRelevantPost: Model.hasOne('mostRelevantPost'),
 
   readTime: Model.attribute('readTime', Model.transformDate),
   readNumber: Model.attribute('readNumber'),