Fix timing issues on convertion migration award emoji

This commit does two things:
1. It adds logic which prevents timing issues when running the
migration. During the migration, notes can be created which _should_
be award emoji and thus migrated. To prevent these timing issues, a
lock is obtained on the table (MySQL) or on Transaction level (PG).
2. There was no down migration before as you'd probably lose some data.
Data effected is all awards on notes. These could be migrated back, as
the noteable type would just be Note, though this would litter the DB
with data which should not be there. This down migration does not yet
delete the table.