Set public email through api

What does this MR do?

Adds the attribute public_email to the create and update user endpoints, such that this attribute can be set by an admin as well.

Note: I also cleaned up the allowed params in the api and in the build service (removed obsolete ones, added missing ones). As a side effect the bug https://gitlab.com/gitlab-org/gitlab-ce/issues/37675. is fixed by this.

The development of this MR is sponsored by @ siemens (/cc @bufferoverflow).

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37675.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21909.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides