Store corpus date as integer

[lukavdplas]

• Stores the corpus min/max date as a year instead of a date (close Min/max data in corpus model: store as integer #1733)
• Changes the input of the corpus filter (on the home page) and the corpus form to use a number input rather than date input
• Changes the component used in the category dropdown on the corpus form, which gets rid of a weird double border

Internal changes:

• The corpus filter component now uses the ReactiveFormsModule for the underlying logic
• The dropdown component now supports [formControl] input.