Phase 2: Order flow: Handle users who are already registered but are not logged in.

[abhinavk96]

To be discussed after Phase 1 (#2000) is complete.

• As per the development in Phase -1, the user is asked to enter his email address.
• If the user enters the email address of an existing user, notify the user, that the email address already exists and he must sign in to continue.
• Handle the logging in of the existing user.
• Also, provide a forgot password link, in case the user does not remember his/her credentials.
  Parent issue Frontend: Implement Ticketing Process as in Eventyay Version 1 #1994