Dropdown menus on edit page

[theosanderson]

For metadata items with a limited range of input options (e.g. country atm), we should display a dropdown instead of a text input on the edit page.

[emmahodcroft]

Just for context, this was feedback from Aine who had put in a country name that isn't accepted by us in the original data uploaded, but then struggled to find the correct name while trying to fix it on the 'edit' page. For example, DRC, RDC, Democratic Repub of Congo.
We could also have predictive text instead.

(And should keep this in mind for any fields we 'control' in future)