Update Helpscout user profile when users change email address

[dd32]

When users change their email address, it'd be helpful if the Helpscout user (if one exists) were updated with their new email (plus their old ones).

This will help keep the teams who use Helpscout aware of the previous email threads related to a user.

There are some downsides to having multiple email addresses listed against a singular customer (Helpscout's name for a contact).

There are some edge-cases where it might not make sense however.
For example, if UserA and UserB both had info@company.org listed as their contact email address at one point, a@company and b@company shouldn't get merged..

If that last point can be accomodated, it'd be good to merge the users automatically.