Allow different registration mails per registrant type

@pfrenssen could you post a description or screenshots of what this change does?

The MR adds vertical tabs to the registrant type edit form so you can override specific notifications depending on the registrant type. For example it allows to send different notifications to users who have paid for a ticket, or won a free ticket, or are registering as a group, ...

I'm not able to test the changes. Adding to my site temporarily using generated patch with 2.0.1 module installed. Do your changes here require some of the changes in your other issues?

My own preference is that it would be nicer to have all the possible registration email settings on the main Registrant Settings page at /admin/structure/events/registrant/settings so that they are all visible and editable in a single place. Easier to compare them and update multiple ones at the same time.

@chrisla that is also a possibility, but this might become unwieldy if a project has many registrant types. It is also quite natural in Drupal to consolidate all options that apply to an entity type inside the bundle edit form (see for example the node type edit form).

I also think it likely that future feature requests will be made to make more of the current options in the Registrant Settings form configurable per-registrant type. For example a customer might want to enable the "Show capacity" option only for in-person attendees, while online attendees are unlimited and should not see the option. And messages might also be something that people want to configure per-registrant type?

The functionality is disabled by default, so existing users should not be impacted. There is an update hook in the MR that disables it for all registrant types.

I'm going to move this to 3.0.x, this is now the main development focus.

pfrenssen → changed the visibility of the branch 3477047-allow-different-registration to hidden.

I ported this to 3.0.x but now I am getting "This value should not be null" validation errors when creating new events.