set email only on user registration, not profile update

[dvoet]

when users create a google account with an external email they have the ability to later change that email. If they do that then update their profile the new email address is saved in thurloe but this is then out of sync with the rest of the system. This PR stops the update, email is only set when user first registers.

Have you read CONTRIBUTING.md lately? If not, do that first.

I, the developer opening this PR, do solemnly pinky swear that:

• I've followed the instructions if I've made any changes to the API, especially if they're breaking changes
• I've updated the RC_XXX release ticket with any manual steps required to release this change
• I've updated the FISMA documentation if I've made any security-related changes, including auth, encryption, or auditing

In all cases:

• Get two thumbsworth of review and PO signoff if necessary
• Verify all tests go green
• Squash and merge; you can delete your branch after this unless it's for a hotfix. In that case, don't delete it!
• Test this change deployed correctly and works on dev environment after deployment

[coveralls]

Coverage increased (+0.1%) to 67.149% when pulling e2f7928 on immutable_email into caec62a on develop.

[davidangb]

👍 changes here look fine. Is there a longer-term story to update the rest of the system so that everything is in sync within Workbench AND in sync with the user's current email address?

[dvoet]

@davidangb no, the thought for the moment is that it is too scary to allow a user's email address to change.