Move static assets source code in related django application folders

For each swh-web django application, create an assets folder in application
one and move related static assets source code (js, css, ...) in it.

Also modify webpack configuration to generate bundles the following way:

  - if swh/web/<app>/assets/index.js exists, a bundle named <app> is generated

  - if swh/web/<app>/assets/<bundle>/index.js exists, a bundle named <bundle>
    is generated

After these changes, some bundles got renamed but also new ones are generated
so update django templates and js code accordingly.

Related to T4398