Feature/blog views

## Blog Views
- Query blog posts by `Category`
- Query blog posts by `User`

## Comments
- `Comment` migration
- `blog#show` includes `comments#create` form and lists blog's comments

## Refactor
- `blog_posts_only` `Post` model scope (was PostType-based previously) for selecting blog posts

## Gems
- `truncate_html` (limit post preview size)