Add `dentistry` as a therapeutic area.
Merge pull request #61 from alphagov/add-dentistry-to-dsu

Add `dentistry` as a therapeutic area.
Turning these single-selects into multi-selects

Since both of these uses are technically multi-selects, we just have a single select option with is "both of the above" --arg!
Merge pull request #62 from alphagov/remove-single-selects

Turning these single-selects into multi-selects