Initial stab at better general datetime picker

Replacement for the old date and time selectors. Opinions invited.

Compared with the old implementation:
+ User can type in the date, instead of using several dropdowns. This makes input faster in many cases (and enables copy-paste).
+ Shows calendar for picking dates
+ Slightly less code on the backend. The date is anyway stored as a string in appropriate format, no need to build it up from the parts.
- Slower to navigate than the simple dropdowns (escpecially for years)
- Currently different formats are not supported. This would be possible to fix with a bit of extra work.
- Requires bootstrap. This could, of course, be changed to something else (like native HTML5 date input) easily.

Currently there's code in the backend for both dates in separate fields and as a single string. It would probably be sensible to pick one or the other anyway, because otherwise you just have extra code paths that receive little testing.