Use post id regardless of how resource url

With FriendlyID, you can use the title, or the actual Post id as part of the
URL to reach a post. The way this was before would fail if an admin had tried
to update a post with tags by posting to /post/the-title/update because
params[:id] would be "the-title" instead of the actual id.

Instead, just use the id we've already look up as in the before_filter.
Recreate posts_tag table with a primary key
Delete matching post_tags in a single query

Otherwise this would have issued N + 1 queries, and also failed if
the posts_tag table didn't have an id (which it doesn't, anymore)