Does two things:
1. Revert "Revert "Add unique constraint to email branding domain""
This reverts commit af9cb30ef3.
2. Don’t allow empty string in email branding domain
Columns with multiple `null`s can have a uniqueness constraint. Columns
with multiple empty string values are not considered unique.
This commit:
- removes any duplicate empty string values
- casts empty strings to null string any time these columns are updated
---
Squashed into this single commits because these two things are not
atomic as individual commits.
notable things that have been kept until migration is complete:
* passing in `organisation` to update_service will update email branding
* both `/email-branding` and `/organisation` hit the same code
* service endpoints still return organisation as well as email branding
