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
