feat(nimbus): Replace message risk question with message review process

[freshstrangemusic]

Because:

• the existing message risk workflow was not meeting our needs:
  • it appeared on every experiment/rollout, regardless of whether or not it contained a message; and
  • it only asked "is this a message?" and did not actually record whether or not the process was followed.
• we do not want to hard gate launching experiments/rollouts with required sign-offs (besides the pre-existing reviewer & Remote Settings checks).

this commit:

• removing the existing messaging risk section;
• adds a new "OMC" user group;
• adds a new "can perform message review" permission flag that is granted to all members of the "OMC" group;
• adds a new "message review" flag on the branches page which requires the "can perform message review" permission to toggle;
• will display a banner warning on messaging experiments/rollouts that have do not have the message review flag set;
• will display a new signoff checkbox in the launch controls when the message review flag is not set; and
• makes some minor cleanups to the launch control JavaScript.

Fixes #16641