[Feature] Add "draft" stage for add-ons

[3vorp]

Is your feature request related to a problem?

Right now, you need to fill out the entire add-on form in one go, or hold onto the browser tab, which isn't really convenient or reasonable.

Describe the feature you'd like

Similar to how the post system lets you save a post without publishing it, the webapp and API should let you save an add-on without it being sent for review.

Screenshot(s)

Notes

There should be probably be less typechecking so fields can be left blank etc

[TheRolfFR]

Does it mean you can have a live version and a draft version of your add-on ? Or is it only for new add-ons ?

This changes drastically the implementation difficulty

[3vorp]

This would be for new add-ons, but I would eventually like to have two collections for add-ons so when an addon gets an update the previous version is still listed on the site until the new version gets approved and replaces it.