MBS-6058: New schema to model deleted/merged entities separately

This new schema allows us to have real foreign key constraints for active
subscriptions, as the bug in MBS-6058 occurs because HumHumXX is subscribed to
artists that don't exist (but also have apparently not been deleted).

The _deleted tables point to artist_/label_deleted rows, which collect lists of
MBIDs that have been deleted, along with the last known name/comment in order
for us to display the deletion clearly in the subscriptions. This data is
probably interesting to customers too, and has uses for us internally (better
404 pages).
MBS-6058: Rewrite deletion/merge code to use new subscription tables
MBS-6058: Rewrite Data layer to have three separate entities for subscription types

It now uses one entity class for subscriptions against entities that are still
'active' (not deleted), another for entities that have been deleted (tracking
the last known name/comment) and another for entities that have been merged.
MBS-6058: Fix subscription emailing script with the new subscriptions model
MBS-6058: Fix the email generation templates
MBS-6058: Factor out the subscribe-on-create logic into a role
Merge branch 'master' into mbs-6058

Conflicts:
	admin/sql/DropFKConstraints.sql
	admin/sql/DropPrimaryKeys.sql
	admin/sql/DropTables.sql
	lib/MusicBrainz/Server/Edit/Artist/Create.pm
	lib/MusicBrainz/Server/Edit/Label/Create.pm
Merge branch 'master' into mbs-6058