Explore date bottom sheets

[kueda]

They're most of the way there, but not quite in line with current designs:

• [x] Show the Date Range option
• [x] TBD: when the user chooses Exact Date, show the Exact Date bottom sheet
• [x] TBD: when the user chooses Date Range, show the Date Range bottom sheet
• [x] TBD: when choosing an individual date... choose a library that looks reasonable and check with Abhas?
• [x] For Date Range bottom sheet, neither date should be allowed to be in the future. If the date picker we use doesn't allow us to control that, add the validation Abhas has specified in the designs
• [x] #1148
• [x] #1153

[kueda]

I don't think this is quite done, @jtklein:

• [x] When choosing an exact date or one of either parts of a date range, the "Confirm" button does nothing
• [x] When you choose months, apply filters, then open Filters again, the months are not checked