Add vertical scroll on Footer menu

[dojutsu-user]

Fixes readthedocs/readthedocs.org#4496

Result:

1. When there are too many versions and translations
   

2. When there are limited versions and translations
   

[jessetan]

This is probably the same as #575

[davidfischer]

I believe this fixes #575 and doesn't suffer from the issues we saw and fixed in #626. In my testing in Firefox and Chrome it looked good and functioned correctly.

I don't want to override @jessetan's opinion though and I'd like to get toward a consensus.

[jessetan]

LGTM

[humitos]

Just a comment.

I think there is only one difference with the current behavior (but not sure if it's browser dependent): in current versions of documentation with alabaster theme the vertical does not appear when scroll is not needed and in the GIF uploaded to this pull request, it does appear even when there is nothing to scroll.

[davidfischer]

I think there is only one difference with the current behavior (but not sure if it's browser dependent): in current versions of documentation with alabaster theme the vertical does not appear when scroll is not needed and in the GIF uploaded to this pull request, it does appear even when there is nothing to scroll.

If I'm understanding this correctly, you're talking about the scrollbar appearing or not. I believe this is browser dependent. Chrome allowed changing some appearance on scrollbars while Firefox did not. However, I believe that is changing.