Add Instance picker to Registration page

[DEVTomatoCake]

Is your feature request related to a problem? Please describe.
Users are currently unable to enter the instance they want to register on on the Registration page, it has to be done on the Login page.

Describe the solution you'd like
Add an instance picker to the registration page, similar to the login one. It should keep the value entered when switching between Login and Register, if possible.

Additional context
PR #144

[habys]

My friends had an issue with this as well and made mistakes on the "Default" server by accident.