`tag` table should not store userid

The tag table stores, amongst others, a userid field.

This userid field needs to be dropped as it has no real purpose and contains incorrect data.

The tag table has a unique constraint containing the tagcollid, and name, but does not consider the userid.

Replication

1. Create two courses and two editing teachers (or above)
2. Edit the first course as the first user, and tag it with something new (e.g. abc)
3. Edit the second course as the second user and expand the Tag section:
   1. Note how there are no suggestions at the moment
4. Enter the same tag (abc) and save changes
5. Inspect the tag table in the database:
   1. Note how there is only one entry for your new tag
   2. Note how that entry has the userid of the first user to use the tag

Why does this matter? Because GDPR.t