Add primary keys to bookmark tables

[itismadness]

These tables do not have a primary key which allows for potential of duplication of data with no real reason for why this might be beneficial.

The tables are: [] boomarks_artists [] bookmarks_collages [] bookmarks_requests [] bookmarks_torrents (this at least has a UNIQUE constraint)

Fix for this will have to assume that these tables have duplicate data and that any Phinx migration must account for this.