Add checkbox option "Only notify about self-registrations"

Problem/Motivation

Checkbox in the "Created"-area in the fieldset, below the roles selection:

Title: "Only notify about self-registrations"
Description: "Only notify about user accounts created by a regular registration. If enabled, no notifications are sent, if the account was created by someone else (admin)"

Taking this further:
We could discuss doing the same for update and delete. Thinking about that leads to the idea, that it might be better to have two checkboxes:

Notify if:
- [ ] User acts for himself
- [ ] Another user crated / updates / deletes the account

That would be more clear and consistent for all the options.

It's a quite typical use case that you might not / only want to be informed about self / admin actions.

Steps to reproduce

Proposed resolution

Remaining tasks

Discuss
Implement
Write tests
Release

User interface changes

API changes

Data model changes