chore: Revert remove eb shift frontend feature flags (#6869)

[tshuli]

• This reverts commit d5bbb4d so that we can continue to use the fallback route
• This is because GSIB attachment uploads via s3 presigned URL are blocked
• Pending a more permanent fix

Tests

Regression

Ensure that email submissions are working

• On GSIB, submit email form with no attachment. Submission should work
• On GSIB, submit email form with attachment. Submission should work
• On GSIB, submit storage form with 2 attachments. Submission should work
  • with .txt files
  • with .pdf files
  • with .zip files

Hotfix cases

• On GSIB, submit storage form with no attachment. Submission should work
• On GSIB, submit storage form with attachment. Submission should work
• On GSIB, submit storage form with 2 attachments. Submission should work
  • with .txt files
  • with .pdf files
  • with .zip files

[KenLSM]

LGTM!