Use admin_post for hooking actions

Instead of hooking actions into the admin page load hook, use admin_post.

It was necessary to introduce some other changes, in order to test functionality.

- Nonce checks for the admin actions.
- Modify how settings are saved for BackUpWordPress and the addons. ( saving add-on settings didn't work )

Original PR was here: https://github.com/humanmade/backupwordpress/pull/534

Fixes #563 and #564 
We should also clear errors at the end of the form, otherwise those errors will continue to show if the user navigates away at this point.