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As Bootstrap 3.x is no longer maintained, migrate swh-web frontend to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