Allow userWithId to be defined per environment spec

Problem: Users cannot achieve their mission with the current UI

Born from #14017 (closed). Currently, users cannot specify userWithId strategy to a specific environment scope. For example, users cannot accomplish this simple task today:

• Enable awesome_feature flag on staging
• Enable awesome_feature flag for a specific user on production

This is a very common usage but we do not support today.

Because of this, we cannot dogfood our FeatureFlag feature as that it's already supported in Flipper and it's actively used in GitLab development process.

Proposal

• Let users set strategy per scope.
• Add strategy option User ids
• Add checkbox to percent rollout to enable fields to add additional ids
• Default checkbox to unchecked
• When checked display textbox to input user ids
• When an environment is toggled off disable all associated inputs
• Remove Target Users section

Scenario	Mockup
All users
User ids
Percent rollout
Percent rollout with additional user ids checked
Disabled
Mobile