Feature/new upgrade page halfway

Simplify the new upgrade page

This way we're able to put the features and the upgrade process together, but benefit from all the error handling that's already in the login & reg pages without a complete rewrite.

This is not proposed as the final solution, but a mid-way point to allow us to test.

@dannyhope + @electricg for your eyes.

- [x] Github redirect works in all cases
- [x] Shouldn't be able to access upgrade page if already pro