Datetime picker updates

Hold off on this until after #136 gets merged and I'll rebase to try to clear out all the extra stuff.

Addresses most of the points in #127. Mostly style/ui-y updates. Minute slider goes up to 60 per the issue, but I don't know if the logic bumps up the hour if it's saved like that.

I haven't done anything yet re: checking if mobile is loading native widgets. Will do so once I can pull it up on my phone.