user: Unify 'listers' and 'loaders' tables into a single 'forges' table

Users do not care about technicalities like the difference between listers and loaders; instead this shows all forges on a single page, which I expect is what users expect.

This also allows providing a single documentation page for everything related to a forge, without duplicating between loader and lister (eg. explanation of how the forge works)

Additionally, this fixes some of the links to swh-web

This MR contains a commit from !351 (merged) and includes the change from !350 (merged)