Fix registration after clicking email link
We weren't correctly jumping into the appropriate bit of the registration
flow when coming in from an email link.
* If we have client secret / sessionId, go straight to registration phase
* Don't reset server URLs when the server type component tells us its
initial value
* Confusingly, pass the custom server URL as 'default server URL' to
the custom server type, as this is what we want the inital section
to be based on.
Fixes https://github.com/vector-im/riot-web/issues/8490
