Description of problem:

When setting the start date, then setting the end date, the start date can then be selected to be after the end date without any warning displayed.
When setting the focus on the start date and leaving the focus, it then disables both date fields and there's no way to undo this unless clicked on cancel button 

Version-Release number of selected component (if applicable):

MTA 6.2

How reproducible:

Always

Steps to Reproduce:

1. Go to Migration > Migration waves
2. Click Create new
3. Select an initial date
4. Select an end date
5. edit the initial date manually using the keyboard

Actual results:

Nothing happens

Expected results:

There should be a warning displayed to the user that the dates are invalid 

Additional info:

When clicking on the initial date field to set focus, and then editing the date manually, both date fields are disabled ( see attached image )