Allow to turn the feature toggle on for all new users

[RachL]

Description

- As a: enterprise user newly created
- On page: /admin
- I want to be able to do: see admin_v3_style as default behavior

To do so, we need to be able to add this group of users to the feature toggle.

Acceptance Criteria & Tests

1. Turn on feature toggle for new users
2. Create an new enterprise
3. See the new admin is here by default
4. Check old enterprise have not changed
5. Check disabling this group of actors brings back the old designs.

[mkllnk]

@RachL, from which date on do you want to count new users? Were you imagining the date of deployment? Or the time the feature is toggled on?

The easiest implementation would be to create a Flipper group which would select all users created after a certain date. That would allow you to enable or disable the feature for the entire group at any time, and still enable for individual users as well.

[RachL]

@mkllnk I assumed each instance could toggle it on or off when they want so yes the flipper group sounds perfect! But we need to define a date and I guess it will be hard coded in the group. So this is block until we know when the first milestone will be done 👍