Added mandatory/optional choice to validators

If a validator is set as `mandatory', then the validation process has to
be run before allowing to click submit button when adding an entity to a
federation. On the other hand, `optional' validator (i.e. mandatory
checkbox is not checked) just produces a warning when the validation
process fail.

More info to come when finished :)
Merge remote-tracking branch 'upstream/develop' into fedval
Add revalidate buttons to queued join requests

Federation join requests awaiting for approval now show enabled
validators (validators are divided into mandatory/optional) as buttons.
After clicking any button, corresponding validator is run and result is
shown.

There's a bug to be fixed. Additional informations is not shown (only
validation result).
Added translatable texts (mandatory/optional validators)
Added description to validator buttons

A validator description is added (as a title attribute to <button> tag)
to every validator in the queue.
Cleaning comments in J_queue.php

Remvoed outdated and unnnecessary comments.
Merge pull request #104 from JanOppolzer/fedval

Added choice to revalidate an entity before approving join request