Add r9k option to boards

[karai17]

r9k mode permanently locks content so that every post must be unique. This includes both files and text content.

This would require a new table in the db to permanently store md5 hashes of images and text in a board so even when content is pruned, it is still permanently blocked from being uploaded again.

{
   integer id
   integer board_id
   varchar md5
}

[shakesoda]

maybe instead of permanent it should prune things over time (and for infinite you just set the time value really high)