Allow configurable redirection URL

[steelbrain]

Some notes about it

• It should be specifiable on the redirect endpoint, ie. not hardcoded in configuration
• It should not allow redirects to any other hosts, so many just &path=/account

I saw this as a ToDo on Docs - Authentication. I couldn't find an existing dedicated issue for this so decided to create one

[deontologician]

@Tryneus is this the same as #508 ?

[steelbrain]

This is similar, but not the same. That issue describes that the redirection url is forced to https thus it doesn't work on http websites, while this issue focuses more on the implementation for a customizable redirection url

[deontologician]

Cool, thanks for the clarification