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.