Refactor dropdowns

[nlhkabu]

To make the dropdowns keyboard accessible, we should follow this guide: https://www.w3.org/WAI/tutorials/menus/flyout/

At the same time, we should fix a bug we found - that sometimes the dropdown are being cut off:

ping @alexwlchan - could you please post your screenshots here? Thx

[alexwlchan]

Here’s a couple more screenshots from our user testing session last Thursday. Browser is Safari 11.0.2 on macOS 10.12.6.

[brainwane]

@nlhkabu About how hard is this to fix -- that is to say, do you think you can fix it this week? I'm trying to figure out when we are going to announce the beta.

[nlhkabu]

@brainwane - I'm a bit behind on this one. Starting tonight. I imagine we could have it merged by early next week.

[nlhkabu]

@brainwane - this is a little more complex than I initially thought. PR is in progress, but probably won't get merged until next week. To be clear, I don't think this needs to block publicising beta - the current solution we have works - just not for keyboard-only users.

[brainwane]

OK, thanks for the update @nlhkabu, moving to a later milestone!