Integrate Msg91 SMS & Email OTP with signup page

[pocha]

• Update firebase functions to actually use the Msg91 API
• Error handling like invalid phone number &/or email should be offloaded to Msg91, which means
  • if Msg91 throws an error, the error need to be shown on the UI
• Other error handling like email/phone mismatch, should be tested. User should be able to see those error messages on the UI
• Only one OTP field need to be shown if the user is found so it will be a login flow.