Add group picture to group detail view

[Encotric]

As a group member, I would like to see a picture of the group I am currently looking at to allow groups to represent themselves better on our platform.

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

Dependencies

• Add group detail view #315

Acceptance Criteria

• A dynamically stored (in a database) picture is shown when visiting the group detail page
• When no picture is found, there should be a simple and minimal placeholder picture describing that no image exists for this group.

[TobiPeterG]

I (Team BA) already started working on this issue.
I already created a branch (no PR), it's based on #377 currently:
https://github.com/hpi-swt2/bookkeeper-portal-red/tree/317-add-group-picture-to-group-detail-view

Maybe some of the changes are helpful