Add group detail view

[Encotric]

As a group member, I want to click on a group name on the „my groups“ page to see an overview page of the group containing the items I can see from this group.

FYI: this story only focuses on the second part of the group detail page - the item overview. There are other stories for the member list and group description and picture as well.

Acceptance Criteria

• Clicking on a group card item on "my groups" redirects me to the detailed view of this group
• Items are shown as described in the wireframe (and/or like everywhere else in the application)
• Only items the group can see are visible

[DieKautz]

I would like to incorporate a small refactor and build an item list component.
This can be done after:

• Remove group entry's grey background of item views #279
• Add notice in my groups page if group has no items #280

are merged