Use the Postgres UUID column type for UUID values

We've been using SQLAlchemy text type for UUID columns rather than Postgres's native UUID column type. (See [here](https://github.com/dssg/triage/blob/f15b86a15484663e948255a5b2251f1c41a38595/src/triage/component/results_schema/schema.py#L169) for an example.) The latter is 1) more efficient and 2) more robust because it handles inconsistent hyphen use.

The biggest drawback to the Postgres type is that it ties triage to Postgres. Are we hoping to remain database agnostic?

Perhaps I'm being optimistic, but I'd expect this to be an easy change, and I'd be happy to do it if there are no objections.