Overhaul permissions

Get rid of Permissible - too complex and inefficient. Replace with:
- a “Locked” trait which works similarly but only evaluates logic on
hydrated models.
- a “VisibleScope” trait which also works similarly but only scopes
queries

This is all we need, Permissible is overkill. There is only one
instance where we have to duplicate some logic
(Discussion::scopeVisiblePosts and Post::allow(‘view’, …)) but it’s
barely anything.

Haven’t decoupled for now, we can definitely look at doing that later.

Permissions table seeder slightly updated.

Also did a bit of a query audit, there’s still a lot to be done but
it’s much better than it was. Some relatively low-hanging fruit
detailed in EloquentPostRepository.

migrations/2015_02_24_000000_create_users_discussions_table.php

@@ -14,7 +14,6 @@ class CreateUsersDiscussionsTable extends Migration
     public function up()
     {
         Schema::create('users_discussions', function (Blueprint $table) {
-        
             $table->integer('user_id')->unsigned();
             $table->integer('discussion_id')->unsigned();
             $table->dateTime('read_time')->nullable();