This adds support for syncing user role via the newly added "role"
attribute, which can be set to either of
['owner', 'administrator', 'moderator', 'member', 'guest'].
Removes durable=True from the atomic decorator of do_change_user_role,
as django-scim2 runs PATCH operations in an atomic block.
These files are not Jinja2 templates, so there's no reason that they needed
to be inside `templates/zerver`. Moving them to the top level reflects their
importance and also makes it feel nicer to work on editing the help center content,
without it being unnecessary buried deep in the codebase.
