Add option to release/deliver queued email for a cPanel user in the UI.

As a cPanel/WHM administrator, I would like the ability to quickly release queued email under a cPanel account via the UI.

While the 'release_outgoing_email' WHM API call does perform this action, there is no UI equivalent to the API call at this time.

Currently, the only UI-based method to deliver queued email is WHM's Mail Queue Manager, which requires searching for individual email accounts to release email. This method can cause delays when attempting to release email for a user's address that has had their outgoing messages held incidentally, such as with a case of a user's address triggering the 'potential spammer' limit in Tweak Settings, or hitting a default account sending limit.

As such, having a UI equivalent to the release_outgoing_mail API call would allow a quicker mail release without the task of releasing held email per address.