Add feature flag to toggle on/off add-forge-now feature
That feature is deactivated by default. If this lands, we'll need to toggle it on for the staging infra.
As an implementation detail, when toggled off, this hides the new add-forge-now menu links. That also plainly deactivates the add-forge-now urls from being served by the server.
Related to T4040
Test Plan
tox, cypress happy (add-forge-now feature enabled for tests)
Migrated from D7370 (view on Phabricator)