Nav Unification: Research incremental rollout mechanism and outline options.

[cpapazoglou]

We’ll create a dedicated support channel for feedback and then roll changes out to 5%, then 20%, then 50%, then 100% of customers, checking in with Happiness at each milestone.

We’ll also show a modal to customers the first time they log in to help orient them to the change.

Relates to #47044
I' ve opened a separate issue regarding the modal #47049

We should probably find a mechanism that doesn't require us to deploy changes. At least not to all platforms (calypso, wpcom, atomic). If deploying changes is unavoidable, we should consider creating an API in wpcom and consuming it from calypso and atomic.

[cpapazoglou]

It seems that internal testing and rollout can be both done by following one of this solutions #47044 (comment)

[cpapazoglou]

Since Internal Testing and Rollout mechanism are interchangeable for this project and it would be also great testing the rollout mechanism as part of Internal testing, I a am closing this in favour of #47044.