swh-web: Migrate frontend to Bootstrap 4.x

As Bootstrap 3.x is no longer maintained, migrate swh-web frontend to
the last major version: 4.x (current version is 4.1).

This induces the following changes:

  * migrate from 3.x css classes to 4.x css classes

  * leverage Boostrap sass variables overriding in order to generate
    custom css for swh-web

  * use the free AdminLTE Bootstrap theme for its clean design
    (see https://github.com/almasaeed2010/AdminLTE/tree/v3-dev)

In adition to that migration process, some cleanup and prettifying
have been performed on Django HTML templates.

Related: T1032