DB migration to unassign deleted tags

[pylipp]

https://trello.com/c/fYH0ezlH

Testing:

• start DB
• list active tags_relations with deleted tags (should be 0):

select count(1) from tags_relations tr
inner join tags t
on t.id = tr.tag_id
where ((tr.deleted_on is NULL) and (t.deleted > 0))

• soft-delete some tags: update tags set deleted = NOW() where id in (45, 47);
• list active tags_relations with deleted tags again (should be 75 rows)
• run the migration. Should print "Updated rows: 75"
• list active tags_relations with deleted tags again (should be 0 rows)
• compare deleted fields (should be the same):

select deleted from tags where id in (45, 47);
select deleted_on from tags_relations where tag_id in (45, 47);

I set tr.deleted_by_id = 1 because I find it inconsistent to have tags_relations rows with non-NULL deleted_on but NULL deleted_by_id.