Add banners as default functionality & display them on mod login / dashboard when enabled (#513)

* Add banner support as a default, integrated option.
Used lainchan's original banner script, authored by barrucadu

* Display banners on moderator login and dashboard

* Remove memes & better directory structure

templates/mod/dashboard.html

@@ -1,3 +1,4 @@
+{% if config.url_banner %}<img class="board_image" src="{{ config.url_banner }}" {% if config.banner_width or config.banner_height %}style="{% if config.banner_width %}width:{{ config.banner_width }}px{% endif %};{% if config.banner_width %}height:{{ config.banner_height }}px{% endif %}" {% endif %}alt="" />{% endif %}
 <fieldset>
 	<legend>{% trans 'Boards' %}</legend>