Cleanup: minor
Cleanup: removed dead references
Cleanup: removed dead code
Wepback: minor refactoring
Upgraded backbone-relational to 0.8.8

Has CommonJS-related fixes which we'll need.
Added some missing requires
Implemented proper assets customization

There's a new `webpack` management command wrapping webpack runtime which
will take care of setting everything up. It will also read a manifest file
from any non-default `STATICFILES_DIRS`, and the entries defined in there
will extend the existing bundles or create new ones.

3rd party code is also able to require modules from the default Pootle
client-side code by using `require('pootle/<module>')`.

Fixes #3314.
Cleanup: removed unnecessary IIFE
Cleanup: minor styling nit