Extend onboarding by more explicit information

.gitlab/issue_templates/Onboarding.md:
Create the ticket as confidential by default (using a short action).
Make the required information in the Details section more explicit and
add entries that are relevant when creating an SSO and/or archweb
account.
Add a note for sponsors of new users, so that they also add a
clearsigned version of the data they provide.
Add a dot at the end of each sentence.
Make the entries for mailing list operations more generic and rely on
the *communication e-mail address*, which may be the user's personal
mail address or a newly created @archlinux.org mail address.
Add warning message about creating a confidential ticket when providing
personal data.
Add checkbox to remind about the removal of personal information,
removal of description history and setting the ticket to be
non-confidential (if it has been confidential due to personal data).
Add checkbox that reminds setting the Team member username to the
@-prefixed username on gitlab (after the user has logged in).