Implemented strong_parameters for Invite/InvitesController.

The email parameter is now required using strong parameters and will throw ActionController::ParameterMissing if it is missing. If the email address is incorrect or invalid, Discourse::InvalidParameters will still be thrown.
Implemented strong_parameters for PostAction/PostActionsController.

PostActionsController now uses strong_parameters' #require to require certain parameters. ActionController::ParameterMissing is now thrown when a reqired parameter is missing, rather than Discourse::InvalidParameters.