Allow Repository & Branches Dropdown's to be navigated via keyboard

[mhutchie]

Describe the improvement that you'd like
Currently the Repository & Branches Dropdown's can only be interactive via the mouse (not via keyboard shortcuts).

These dropdowns should be brought inline with the CustomSelect implementation (used on Dialog's), which fully supports the keyboard navigation and selection of options. (e.g. arrow up/down, enter, space).

Additional context (optional)
This request was split out of #479, so it could be delivered independently.