Add description to group detail page

[Encotric]

As a group member, I would like to see a description of a group when I visit its detail page. The description should be a text block of a maximum of 2000 chars.

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

Dependencies

• Add group detail view #315

Acceptance Criteria

• A dynamically stored (in a database) description of a group is shown when visiting the group detail page
• Group description can't be longer than 2000 chars.

[TobiPeterG]

tracked in #377