Refactor AdminMailer and add default mailer style

* Rename PaymentMailer to AdminMailer and update naming/namespacing
* Pass args to AdminMailer by ID instead of object
* Rewrote the email, added rewards text, and implemented a default email style
Refactor admin header nav
Add admin payment notification toggle
only send payment email to users who have payment notificaitons enabled
Add before_create callback to set default payment notification email to
enabled
Fix error in admin payment mailer logic
Fix bitwise logic in remove_email_setting
Dont persist flash message in notifications pane
Add text specifying email notification