Description of problem: No validation for 'Source repository` field when creating new custom migration target

Version-Release number of selected component (if applicable): 6.1.3 Stage

How reproducible: 100%

Steps to Reproduce:
1. Go to Administration => Custom migration targets
2. Create a new one
3. Fill in fields but leave empty 'Source repository'

Actual results:

 There will be no red text below the field notifying that field is mandatory and empty

Expected results:

Red text below the field should appear  

Additional info: