Allow users to use alternative identities for commits made online (BB-9432)

Now when user does online edits / merges, email address used for record such commits is the "primary" one. That address is however used also for account related notifications and mainly for login and password recovery.

Hence, by doing online edits this email address is compromised.

This could be prevented if user had the option to instruct system to use other verified email address, preferably one matching his regular public Git identity.