swh-web: Improve overall design

This commit improves the overall design of swh web applications by
making it more compact in order to gain more space for displaying the
archive content.

The following changes have been applied:

  - use a collapsible sidebar for switching between the different
    features (currently Web API, Browse, Search, Vault, Help) instead
    of putting them in the top navbar

  - remove the use of panels (cards in Boostrap 4.x) as the main layout
    for swh-web pages, instead move their headers in the top navbar and
    use their content as the main page layout

Closes T1052
Closes T1032