Adding a spoiler tag

This is an implementation of the following feature request: http://www.reddit.com/r/ideasfortheadmins/comments/oqnxk/on_behalf_of_the_entire_book_and_tvshow_subreddit/

This adds a "spoiler" tag, which acts the same way as a NSFW tag: submissions can be marked as being spoilers, and get a new thumbnail and a stamp. A submission can be marked as both being a spoiler and being NSFW (NSFW taking precedence).

The spoiler setting can be changed on a per-community basis: if it's not enabled, submissions cannot be marked as spoilers (it is off by default). When enabled, users also have the ability, through a checkbox in the sidebar, to enable or disable the display of spoilered submissions (just like they can choose to hide NSFW submissions, except this works on a per-community basis).