add unique indexes to tables columns that require unique values

https://osraav.atlassian.net/browse/OSRA-252
OSRA-252 #code-review
Additionally to use statuses as an example.
Theoretically it will not be written to any more so there is no performance hit on a write.
Postgresql's query planner is clever enough to know that a single item represents 25% of the rows and so it issues a sequential scan ignoring indexes anyway because it knows that this will be quicker.
Adding the indexes gives us an additional guard than no one can add another row with a code or name that is the same inside or outside of the application.
I can't see a reason not to have it.