This PR is in preparation of, but independent of, an upcoming suggestion for a feature addition:
* The first commit moves a tiny bit of logic into a separate function to prepare for extension of that logic, avoiding duplication
* The second commit moves checking for disabled registrations earlier, which, I think, has merits in terms of performance and resilience (hopefully not significant, but who knows?)
* The third commit adds simple unit tests for SignUp() and SignUpPost() to avoid the long-ish roundtrip over integration tests
* The forth commit introduces `ctx.Data["DisableRegistrationReason"]` for the signup template to use as the reason printed if `.DisableRegistration` to prepare for other reasons to be added
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10915
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Nils Goroll <nils.goroll@uplex.de>
Co-committed-by: Nils Goroll <nils.goroll@uplex.de>
