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.