Top scores: use timezone-aware datetimes when querying

Fixes #248.
runner: Properly forward all unknown commands to django-admin.py
runner: Saner yes/no handling.
runner: Use a non-conflicting string formatting the random SECRET_KEY
runner: Exit with code 2 when we are prevented from overwriting the config file
Contributions: make sure to account for system user submissions too

Submissions made by the system user were being ignored altogether and
there was no way to access them in the editor or see the relevant numbers
in the detailed stats page.

Fixes #253.
CacheAnonymousOnly: checked if `user` is available in the request

Fixes #255.
User admin: created initial layout skeleton

This is just the scaffolding, nothing fancy yet.

Fixes #230.
AjaxResponseMixin: clarified docstring
User admin: enabled internal XHR API

This simply enables the endpoint with the basic methods. Extra tweaks will
be done as part of integrating the front-end.

Fixes #231.
XHR API: also return object count for collections
Cleanup: moved BB-related libs to `vendor/backbone/`
XHR API: implemented keyword-based simple field filtering

When the `search_param_name` GET parameter is set, its value will be used
to filter the collection to be returned.
User admin: enabled search via API
User admin: enabled form-based data validation

Handles the special situation for passwords, where it needs to call the
`set_password()` method instead of storing the provided value as-is. It
also requires the password field when creating new users.
User admin: validate LinkedIn URL
Backbone.js: upgraded to 1.1.2
Underscore.js: upgraded to 1.6
Webpack: added client-side module system

Client-side code configured to use Webpack can now enjoy
CommonJS/AMD-style modules at the cost of an extra build step. Refer to
the docs for further details.

Fixes #251.
User admin: implemented new front-end

Fixes #232.