As it was decided that the name "tags" would be better for the clarity
of the feature, all files and code named "cluster" or "worker cluster"
have been removed and replaced with "tag" and "worker tag". This is only
a name change, no other features were touched.
This addresses part of #104204.
Reviewed-on: https://projects.blender.org/studio/flamenco/pulls/104223
As a note to anyone who already ran a pre-release version of Flamenco
and configured some worker clusters, with the help of an SQLite client
you can migrate the clusters to tags. First build Flamenco Manager and
start it, to create the new database schema. Then run these SQL queries
via an sqlite commandline client:
```sql
insert into worker_tags
(id, created_at, updated_at, uuid, name, description)
select id, created_at, updated_at, uuid, name, description
from worker_clusters;
insert into worker_tag_membership (worker_tag_id, worker_id)
select worker_cluster_id, worker_id from worker_cluster_membership;
```
Avoid users of the persistence layer to have to test against Gorm errors,
by wrapping job/task errors in a new `PersistenceError` struct.
Instead of testing for `gorm.ErrRecordNotFound`, code can now test for
`persistence.ErrJobNotFound` or `persistence.ErrTaskNotFound`.
