[Discussion only] First attempt on a more imperative approach

**do not merge** - this commit will be added on a PR with the implementation

Changes done from the original idea (original idea at [ezsystems/BehatBundle/Features/*](https://github.com/ezsystems/BehatBundle/tree/master/Features)):
- move from a strait declarative way to a declarative/imperative way
- update feature description to match the actual User Stories
- merge the anonymous/administrator users to same feature file since the user (actor) is a developer

Declarative and imperative way
Since the above scenario already have all needed for the create content type group request, we don't need to repeat it here, so we change from imperative way:
```
        When I create a "POST" request to "/content/typegroups"
        And I add "content-type" header with "Input" for "ContentTypeGroup"
        And I add "accept" header for a "ContentTypeGroup"
        And I make a "ContentTypeGroupCreateStruct" object
        And I add "some_string" value to "identifier" field
        And I send the request
```
to declarative way:
```
        When I create a Content Type Group with identifier "some_string"