Basic change email functionality

[#76898458] https://www.pivotaltracker.com/story/show/76898458

Signed-off-by: Rob Gallagher <rgallagher@pivotallabs.com>
Handle invalid code when verifying email change.

[#76898458] https://www.pivotaltracker.com/story/show/76898458

Signed-off-by: Rob Gallagher <rgallagher@pivotallabs.com>
Handle username conflict when changing email

[#76898458] https://www.pivotaltracker.com/story/show/76898458

Signed-off-by: Rob Gallagher <rgallagher@pivotallabs.com>
Merge branch 'feature/change_email' into develop