Branch merge of change-tagging branch with trunk

Branch merge of change-tagging branch with trunk
-- Introduce tagging of individual changes (revisions, logs, and on recentchanges). The tags are customisable, and currently settable by the Abuse Filter and by the TorBlock extension. The tags can be styled on the various pages on which they appear.
-- Introduces a schema change, three new tables (valid_tag, change_tag, and tag_summary).