Webmail and user auth flow fix

[asutoshranjan]

• each account is being linked to a particular phone no. and particular roll number/ webmail id logging with another webmail id throws auth error

• fix by verifying user's roll number for webmail login

• Phone number input page

• remove deregister webmail from menu

• Phone no. already registered case error in login

• webmail login screen if the user is signed out

• redirect to event website for non - student registration