[Feature flag] Rollout of mobile_push_registration_api

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Label this issue

Rollout issue for the mobile_push_registration_api feature flag (mobile push notifications for to-dos; part of the stacked MR series adding APNs push support for the GitLab mobile app prototype).

• Flag type: development, actor: user, default off.
• Enable for dogfooding users first, then group actors, then globally.