admintools: add missing stub for `bans_for_domain_parts`

**Note to the admins** -- to reproduce the error that this patch fixes, you need to run reddit without the non-opensourced `r2admin.models.admintools` module imported through [here](https://github.com/reddit/reddit/blob/master/r2/r2/models/admintools.py#L437-L440). This bug only manifests itself when executed without that module.

So, because `r2.models.admintools` doesn't define `bans_for_domain_parts`, submitting a link causes the app to throw this exception:

    sys.path: ['/usr/local/bin', '/home/reddit/reddit/r2', '/usr/lib/python2.7/dist-packages/PIL', '/usr/lib/pymodules/python2.7', '/home/reddit/reddit-i18n', '/usr/lib/python2.7', '/usr/lib/python2.7/plat-linux2', '/usr/lib/python2.7/lib-tk', '/usr/lib/python2.7/lib-old', '/usr/lib/python2.7/lib-dynload', '/usr/local/lib/python2.7/dist-packages', '/usr/lib/python2.7/dist-packages']
    Module r2.lib.validator.validator:236 in newfn          view
    >>  simple_vals, param_vals, *a, **kw)
    Module r2.lib.validator.validator:306 in validatedForm          view
    >>  *a, **kw)
    Module r2.lib.validator.validator:285 in _validatedForm          view
    >>  val = self_method(self, form, responder, *a, **kw)
    Module r2.controllers.api:379 in POST_submit          view
    >>  filled_quota = c.user.quota_full('link')
    Module r2.models.account:583 in quota_full          view
    >>  limits = self.quota_limits(kind)
    Module r2.models.account:577 in quota_limits          view
    >>  if self.cromulent():
    Module r2.models.account:567 in cromulent          view
    >>  if self.has_banned_email():
    Module r2.models.account:554 in has_banned_email          view
    >>  which = self.which_emails_are_banned((canon,))
    Module r2.models.account:543 in which_emails_are_banned          view
    >>  from r2.models.admintools import bans_for_domain_parts
    ImportError: cannot import name bans_for_domain_parts

This pull request adds the missing stub for `bans_for_domain_parts` to fix this issue.