Implement UI for recording manual exchanges; #53

Sometimes we do manual payouts (maybe payins someday?) and we need a way
to record those in the database so we don't have to keep doing them
directly in SQL(!). This adds a form to the history page for site admins
to use. It extends the exchanges schema to keep track of who made manual
adjustments and why.
Bump version to 7.1.0
Bump version to 7.1.0-dev