Implement legacy backoffice extension

See http://jira.ez.no/browse/EZP-22210.
Adds a custom "Comments" content tab to the legacy backoffice.
Requires https://github.com/ezsystems/ezpublish-kernel/pull/719.

![legacy backoffice comments](https://f.cloud.github.com/assets/235928/2169397/4c6213a6-9559-11e3-8076-20f28e11a866.PNG)

Note: adds a requirement to a new  bundle, http://github.com/ezsystems/ngsymfonytools-bundle.