Add signal (pretalx) for state change

[johanvdw]

We have some custom checks when a state change. These are now in our branch of pretalx.

We should create a signal in pretalx and listen to that and check:

• that the change is made by a devroom manager of that track

[johanvdw]

looks like a very related signal already exists:

https://github.com/pretalx/pretalx/blob/main/src/pretalx/submission/models/submission.py#L387C1-L392C14

submission_state_change

but this is after the change is done, I would like to have one before

[johanvdw]

upstream issue: pretalx/pretalx#1974