Fixed package path in testsetup.

The testrunner wronlgy searched the complete project, so also the `src` directory for tests, which end in import errors. With this change the runner only runs test from the opengever.core package.
Fixed backref name of the groups_user join table.

See https://github.com/4teamwork/opengever.ogds.models/pull/4 for more details.
Added missing kgs inclusion.

The commit #9cba578e9105ab15ab20f58179e4563db9786ca9 dropped the
opengever-zug kgs inclusion, which is right, but it dropped the
complete inclusion of the kgs version. The standard opengever
version pinings are necessary.
Merge pull request #236 from 4teamwork/pg_add_misingkgs_inclusion

Added missing kgs inclusion.
Merge pull request #233 from 4teamwork/pg_fix_backref_name

 Fixed backref name of the groups_user join table
Removed IMailInAddress behavior. Not needed any more with ftw.mail 2.0.
Provide an IntIdResolver adpater to generate Dossier Mail-In addresses

based on IntIds, and use it instead of the old implementation.
Removed og.mail IMailSettings registry interface (now done in ftw.mail).

Added upgrade step to migrate settings to ftw.mail's IMailSettings and
remove old og.mail IMailSettings.
Hide standard ftw.mail mail-in viewlet (we display the mail address in the byline).
Update mail related tests.

- Mails can't be added inside tasks. Neither directly nor via mail-in.
- Expected mail domain should be set in test itself
Use ftw.mail from source until we create a KGS that uses >= 2.0.
Merge pull request #232 from 4teamwork/lg-bump-ftw-mail-to-2.0

Bump ftw.mail to 2.0
Task overview: Hide client prefix in issuer value for a one client setup.

Additionaly I added tests for the different possibilities for the issuer display.