Image name conflicts - fixes #608

WorkingLanguage was used to make names unique across multiple languages
Add method to check if a user is in a certain group

This is an extension to the api that the groups module provides to other
modules. In some modules it can be convenient to check if a user is in a
certain group.

I already submitted this once, but was asked to resubmit for 3.7, see
https://github.com/forkcms/forkcms/pull/698
Merge pull request #758 from ikoene/blog-image-name-conflicts

Image name conflicts - fixes #608
Add missing items in changelog.
Fix merge conflicts
Merge branch 'jonasdekeukelaere-fix-contentblock-positions'
Merge pull request #777 from turanct/is-user-in-group-3.7

Add method to check if a user is in a certain group
Prepare for release 3.7.1
Fix changelog layout
Fix array to string conversion in MailMotor.