phabricator: model: Reference the forge's instance name in model

As phabricator is an "instance" lister (there exists multiple instances of phabricator in the wild), we need to reference that information.

In effect, this aligns phabricator lister with for example the gitlab one.

Related #1801 (closed) Related swh/meta$431 (db migration script)

Annex:

• models: Unify tablenames using singular as main archive's convention (swh/meta$431)

Test Plan

• tox
• docker
  • up (without diff): test swh/meta$431 ~> ok
  • down
  • up (with diff): new columns populated

Migrated from D1594 (view on Phabricator)