Add tags table

[rhystmills]

What does this change?

Currently, tags are modelled as a comma-separated string on each rule. Modelling them in a separate table will make working with them simpler, for example modifying tags that are shared across many rules, or pulling a list of all unique tags.

This PR adds:

• a tags table
• methods to add, update, and remove tags
• tests for those methods

It does not add a rules join table (though that will be the next piece of tags work) or add any means of managing tags to our frontend.

How to test

1. Run the application locally according to the instructions in the readme. Does it build? (This will show that the DB evolution has succeeded)
2. Run the tests (sbt test). Do they all pass?

[jonathonherbert]

Ace! A few non-blocking suggestions.