Draft: New website template and data source

The original HTML generator is still present in this branch; it can be accessed with swh dataportal generate. To use the new one, you'll need to use swh dataportal build instead.

• integrate bootstrap to share the look & feel of the archive
  • use swh main colors
  • avoid a div soup
  • optimize css (with an optional purgecss step)
  • organize a bit datasets by usage instead of just types
• write a data.yml file and associated models to ease datasets update
  • data.yml
  • models
• copywriting
  • website intro
  • license / citation
  • datasets descriptions

A demo is available at https://datasets.softwareheritage.org