Fix gmail auth dialog popping up over and over again.

Summary:
This happened when entering "david.faure" as username in the
account dialog, without "@gmail.com".

Normalize gmail username so that it matches the JSON account info
returned by GMail authentication.

Test Plan:
Entering david.faure as username and saving,
leads to gmail.com appended (both in the auth dialog and when reopening
the account configuration dialog).

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14852