Form validation should include making sure that start dates are before end dates

[bonniegee]

Is your feature request related to a problem? Please describe.

Currently, there is no check in form validation on whether the end date is before the start date. This is a possible user error that should be caught before the form is saved.

Describe the solution you'd like

Before a rights shell is saved, make sure that the following condition is met for basis dates and any act dates:

• The end date is not before the start date

I think this should only compare the start and end if they're of the same type (i.e., a number of years or a date).

Additional context

This is rescoped from #67.