Add migration to change case of tags

Add migration to change case of tags

This migration does not support SQLite as long as this engine does not
support Unicode in LOWER().

This migration starts by retrieving the list of lowercase tags which
need to be migrated. Then it retrieves the list of tags for each tags
from the previous step in order to migrate entries. It handles deletion
of empty tags. At the end the migration makes a full scan to update the
label of all remaining tags.

WARNING: THIS MIGRATION IS IRREVERSIBLE.

Signed-off-by: Kevin Decherf <kevin@kdecherf.com>