Drop Revisions model (wincent.dev, b6312eb)

My initial wiki implementation only had a "revisions" notion because I unthinkingly modelled it after other wikis. In reality I don't need it and it is an unecessary complication, so remove it. (This is not a community-driven wiki, nor one which allows anonymous edits, so the ability to go back in time is not as important).

Signed-off-by: Greg Hurrell <greg@hurrell.net>