Move form definition retrieval out of controller constructors for proper access checking in other contexts

[jensschuppe]

Fixes #14. Replaces #15.

This is a more comprehensive approach to moving form definition retrieval out of controller constructors.

The \Drupal\civiremote_event\Controller\RegisterFormController class now responds to all registration-related routes (register, update, cancel) and includes a combined access check that can be used reliably outside of those route contexts (e.g. when validating a URL for displaying on another page).

Also, retrieving the form definition is now done in a centralized controller method before handing things off to the respective form builders, which do not have to care for initializing things in their constructor anymore. This also prevents intermittent error messages be displayed for errors during route access checks.

@FeyP Would you be able to give this a try and leave a review?

[jensschuppe]

@dontub if you have time, could you have a look as well? Also, the PHPStan jobs are failing due to failing GitHub authentication …

[jensschuppe]

Thanks @dontub I did the changes you requested in 41052ad - care to have a quick look again?

[jensschuppe]

This introduced a regression causing all forms for all contexts (create, update, cancel) to use the RegisterForm form builder class instead of the resprective subclasses. #24 fixes that.