Add return URL to account activation email link

[Oanh-and-only-Oanh]

Current behavior:
When a user creates a TC account, the account activation email link is static which mean all new users to directed to https://api.topcoder.com/pub/activation.html?code=2GN2IW81KTM&retUrl=https://www.topcoder.com/community/practice

See screenshot of activation email with status URL

Expected behavior:
When a user creates a TC account, their account activation email link should be a return URL of wherever they came from. That way when they activate their account, they will be redirected to their last URL.

Use cases:

1. When user is not logged in,
   If they click on a "Login" button on the Community side
   Then they are directed to the signon page.
   If they click on "Sign up" for an account button, their return URL is dropped.
   
   We need the return URL to be persistent as a place market for the user to go back to when they activate their account.

2. When user is on Marketing site www.topcoder.com
   If they click on Sign up button as a "Member"
   Then the account activation email link should direct them to topcoder.com/start