[forms] return to event when form was submitted

[Jelmer van der Linde]

This works correctly if no error is made while filling in, but if there were a few retries, the return url is probably lost.

[Jelmer van der Linde]

Return to original form when sign-up completed

Also don’t lose the member information. This fixes issue #80.

→ <<cset 5e4c76fc7e4e>>