Date picker text entry

[thatoddmailbox]

It should be possible to type into the date picker somehow. Right now, tabbing between the date picker and any other field works, but you can't actually do anything to the value when the date picker is focused. This affects both the homework and the calendar event modals. (although it is less important for homework due to quick add)

I think we either should allow using the keyboard to open the drawer and select a date (with the spacebar and arrow keys?), or allow typing in a date (which would require some work to handle different formats and do something logical for invalid values)