Orders: Make flow similar to eventyay Phase - 2

[abhinavk96]

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the Upstream development branch.
• The acceptance, integration, unit tests and linter pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Changes proposed in this pull request:

• 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.
• Handles the logging in of the existing user.
• Also, provides a forgot password link, in case the user does not remember his/her credentials.

Fixes #2001

[abhinavk96]

To be tested/merged after fossasia/open-event-server#5549