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

src/Api/Serializer/BasicDiscussionSerializer.php

@@ -84,9 +84,9 @@ class BasicDiscussionSerializer extends AbstractSerializer
     /**
      * @return \Tobscure\JsonApi\Relationship
      */
-    protected function relevantPosts($discussion)
+    protected function mostRelevantPost($discussion)
     {
-        return $this->hasMany($discussion, BasicPostSerializer::class);
+        return $this->hasOne($discussion, PostSerializer::class);
     }
 
     /**