Remove Category model, replacing with parent/child relationships on the Forum class.

TODO: Top-level forums should be categories, but for now we have an explicit is_category boolean - this is because I'm not sure how to get the 'parent' form to contain a "Null" option.  Once we can get a "Null" option in the parents list, we can replace the is_category boolean code with "parent is None" checks.
Subforum-related template tweaks.

Categories are now just interpreted as forums, there is no category-specific view.
Forums on "index" view list their immediate subforums; same with subforums on "forum" view.
Forum model has helper function "get_breadcrumbs", that navigates up through the parent hierarchy and returns a list of all the parents - top-most parent [category] first through to Forum's immediate parent last.
Cleanup subforum changes

Fix example data creating topics in categories.
Don't render "posts" table on categories, and hide the new topic button.
Added a check to new_topic that redirects requests away if the new_topic's forum_id is a category.
Replaced some model properties with db queries, first step in moving them into caching layer rather than db layer.

Replaced properties:
- `Topic.post_count`
- `Topic.first_post_id`, `Topic.first_post` -> `Topic.first_post`
- `Topic.last_post`, `Topic.last_post` -> `Topic.last_post`
- `Forum.post_count`
- `Forum.topic_count`
- `Forum.last_post_id`, `Forum.last_post` -> `Forum.last_post`
- `User.post_count`
Caching implemented for post / topic_counts.  Still got to figure out how to best cache last_post
Figured out how to cache last posts; we had to load topic and user before we cached cuz they were dynamically loaded relationships.
Ability to lock forums