Keyboard navigation should be available to get into the Compare Versions drop down menu

[ioanarusiczki]

Followup for #757

STR:

1. Load https://code.addons-dev.allizom.org/en-US/browse/618459/versions/1689844/
2. Use keyboard navigation (Tab) or the downwards arrow to get to Compare Versions button
3. Press Enter then try to go using the keyboard to Old version
4. Observe which is the next element highlighted

Expected results:
Old version from the the drop down menu is highlighted.

Actual result:
Comment Summary button is selected.

Notes:
If I open the menu and then I click inside it the keyboard navigation can be done.

┆Issue is synchronized with this Jira Task