Added choice to revalidate an entity before approving join request

Added a checkbox (Mandatory) to Federation validators to distinguish mandatory and/or optional validators.

The idea is that if a mandatory validator fails, metadata won't be allowed to submit. However, this isn't implemented yet. (Disable submit button using JS? Hide submit button using JS? Another solution...?)

Now, a join request in the queue shows mandatory/optional validators. Description is available upon pointing the mouse to the button which has the name of the validator. After clicking the button, corresponding validator is run. (Addition information isn't shown... yet... Don't know why, I'm missing something.).

More to come soon :)