Develop a "Add forge now" mock-up

Let's do a mock-up of the functionality.

That should help in bootstraping this:

• a bit of code to actually have some views to show/play with [1]

• take some screenshots to document alongside

• exchange on what's not so great and can be improved

• finally make it evolve into the real thing (with tests and all)

• [1] opt-in in staging, out in production for example

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Migrated from T3733 (view on Phabricator)