Add database fields for string priorities (issue #123)

Signed-off-by: Michal Čihař <michal@cihar.com>
Remove unused imports from migrations

Signed-off-by: Michal Čihař <michal@cihar.com>
Use named args which we expect

Signed-off-by: Michal Čihař <michal@cihar.com>
Automatically adjust units priority when source string priority changes

Issue #123

Signed-off-by: Michal Čihař <michal@cihar.com>