Provided defaults for *_URLs that work with i18n.

This can be done only if mezzanine.accounts is installed (as then we know
that its urls.py is included); contrib.auth has similar views / pattern
names, but even if it's installed its urls.py may not be processed.

The *_URL settings can take a couple of different things as a value:
* plain string (Django's default) -- these cause decorators such as
  @login_required to redirect to the default language version of the page;
* translation proxy (e.g. _("/en/login/")) -- that's better, but causes
  translation redundancy that can be a source of errors (translations of
  url patterns and *_URLs need to match);
* view name -- works, but is deprecated and makes it easy to create
  redirection loops (uninstalling the app and forgetting to update
  something may show up as a loop rather than a reversal exception);
* pattern name -- the best, though the name needs to be defined in some
  included urls.py (also for testing).