feat(form-v2/react-to-angular/3): remove infoboxes when rollout percentages hit thresholds

[wanlingt]

Solution

Remove visibility of switch-back-to-AngularJS infoboxes after rollouts hit their respective % (10% for public respondents, 100% for admins)

Closes #3933

Details

• Add
  respondentRolloutEmail, respondentRolloutStorage, adminRollout to frontend end vars
• Determine showSwitchEnvMessage boolean depending on whether the rollout % > the % threshold to remove the infobox

Breaking Changes

• No - this PR is backwards compatible

Tests

• Set the rollout % for an email mode form below the % threshold for removal of the infobox. Infobox should be visible.
• Set the rollout % for a storage mode form below the % threshold for removal of the infobox. Infobox should be visible.
• Set the rollout % for admins below the % threshold for removal of the infobox. Infobox should be visible.
• Repeat the 3 tests above, but set the rollout % to be above the % threshold for removal of the infobox. The infobox should no longer be seen.

[timotheeg]

lgtm