Django-arcutils

- Added official support for Python 3.5.
- Dropped official support for Django 1.7.
- Started adding support for Django 1.10.
- Improved default WSGI script.
- Started using tox to test multiple Python/Django combinations.
- Fixed up some settings-related stuff (low level internal stuff that
likely has no observable effect).
- Added default `AUTH_PASSWORD_VALIDATORS` because we shouldn't allow
users to have terrible passwords.
- Fixed Bootstrap stylesheet link in foundation.html.
- Upgraded DRF 3.4 => 3.4.7, which required some internal changes to
our `TemplateHTMLContextDictRenderer`.
- pytz is now included as a default dependency.
- Upgraded ldap3 1.2.2 => 1.4.0.

Revert back to using StaticFilesStorage by default in staging.

Added

- Added `PrefixedSettings`. This provides a nice way to specify defaults
for a group of settings (e.g., for a Django app or package) and to get
access to a group of settings.
NOTE: There's more detail about this in the `Changed` section below.
- Added `SECURE_PROXY_SSL_HEADER` to default stage local settings.
This is needed (only) in staging because the main Apache instance
proxies to app-specific Apache instances.

Changed

- Constrained Django version depending on Python version: 1.8 is
installed on Python<=3.3; 1.9 is installed on Python>=3.4.
- Disabled all logging in default test local settings by using a
`NullHandler`.
- `clearsessions` is now run nightly by default (when the `wsgi.py`
template provided by ARCUtils is used).
- Replaced `make_prefixed_get_setting` function with `PrefixedSettings`
class; the latter provides the same functionality, but is perhaps
easier to understand, and its usage is more similar to regular Django
settings (it provides dict-like access to the prefixed settings).
NOTE: This is a breaking change, but `make_prefixed_get_setting` has
so far only been used internally in ARCUtils. `PrefixedSettings` is
more suitable for use externally.
- Removed unused `_suffix` arg from `CASBackend._validate_ticket`.

Fixed

- Corrected import path to `NestedObjects`; Django standardized all
utility module names in 1.7 (`util` => `utils`).
- Made Django's `ManifestStaticFilesStorage` the default in stage local
settings. Not sure what the rationale was for this being different
from production before.

- Corrected default ARC.cdn.paths.jquery-js setting.
- Added ARC-specific Django Admin stuff (added top level `admin`
module).

Fixed

Fixed creation of users when using CAS auth. Previously, the `username`
attribute wasn't being set; now it is. This regression was introduced
when the use of CAS callbacks was deprecated in 2.8.0.

Upgraded django-local-settings from 1.0a17 to 1.0a20. 1.0a20 is
*backward incompatible* with 1.0a19 in that it uses `{...}` for
interpolation groups instead of `{{...}}`. In addition, since
`str.format()` is no longer used to do interpolation, `{{X[y]}}` won't
work now; you have to use `{{X.y}}` instead (which is actually a *good*
thing).

This change also means that this version of ARCUtils is backward
incompatible, but it's not *majorly* incompatible, so I don't think
bumping the major version is appropriate in this case.