Welcome message for invited users

[jace]

When a user is invited to have an account and they accept, Lastuser needs to ask them for a password so they can access the account again.

At this point Lastuser should check with LoginProviders such as the Google login provider if they can identify the email address as connected to an external service (for instance, @gmail.com addresses or email domains with MX pointing to Gmail servers), and if so, should inform the user that setting a password is optional and they can login with the external service provider instead.

Caution: this is not failsafe if, for example, the user is using a Google Apps domain alias as only the original email address is reported when they attempt to login with Google. Case in point, hasgeek.in vs hasgeek.com.

This issue complements #84.

[jace]

Closing this issue since there's no failsafe and generic way to implement it.

[jace]

Re-opening as the cautionary note listed here does not arise if the invited user (a) first opens the invite email, thereby identifying themselves to Lastuser, and (b) then clicks on a Google login button, thereby linking UserExternalId to this Lastuser account.

[jace]

mxsniff can be used to identify Google Apps domains.