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/Discussion/Discussion.php

@@ -382,6 +382,16 @@ class Discussion extends AbstractModel
         return $this->belongsTo(User::class, 'last_user_id');
     }
 
+    /**
+     * Define the relationship with the discussion's most relevant post.
+     *
+     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
+     */
+    public function mostRelevantPost()
+    {
+        return $this->belongsTo(Post::class, 'most_relevant_post_id');
+    }
+
     /**
      * Define the relationship with the discussion's readers.
      *