2. send checkbox value instead of a hardcoded value

2. added new function check() to enable/disable submit button based on status of checkbox

2. fixed overlay message text
3. fixed error message broken href tag

 -  Issue: in case referrer was null, app redirected to /None
 -  Fix: in case of redir value in request, set session to redir value
         else, set session to default referrer (from vars.py))

added flask-cors and used allow cross_allowed_origins to fix the issue seen on prod cluster (400 CORS error)

sending text message for welcome message, error message and cancel message as a variable from messages.py