Add LibreJS compliance for swh-web

This commit enables to browse swh-web with its JavaScript features enabled while
using the LibreJS Firefox plugin (https://www.gnu.org/software/librejs/index.html).

The compliance is obtained through the following:

• Generate a WebLabels HTML page through the help of a homemade webpack plugin referencing all the generated JavaScript assets we distribute. For all assets, all bundled source files in it are referenced along with their license and a link to their non-minified source code.
• Slightly modify some Django templates to state the license of scripts directly included in the HTML.

Closes T1512