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

migrations/2015_02_24_000000_create_posts_table.php

@@ -36,8 +36,9 @@ return [
             $table->engine = 'MyISAM';
         });
 
-        $prefix = $schema->getConnection()->getTablePrefix();
-        $schema->getConnection()->statement('ALTER TABLE '.$prefix.'posts ADD FULLTEXT content (content)');
+        $connection = $schema->getConnection();
+        $prefix = $connection->getTablePrefix();
+        $connection->statement('ALTER TABLE '.$prefix.'posts ADD FULLTEXT content (content)');
     },
 
     'down' => function (Builder $schema) {