Add special NSFW and spoiler tags
Fixes #18 (closed).
I added two special tags for NSFW and spoiler posts:
The special tags are brought to the front of the tags array when a post is submitted.
I also added a client-side script that brings the special tags to the front when you're creating a new post, although I'm not sure if it should stay in as re-ordering the tags while the user is putting them in could be a bit jarring (though it only runs when the user deselects the tag input field or presses enter, which helps a bit).