[server] Add group photo functionality

[Kuchteq]

• Add a new field to openapi.yml file that is the path to a group cover photo. Given that the group photo can be big, we don't want to keep it inside the sqlite database itself like we do with the user photos.
• On group create, POST /groups generate a UUID as a path to the group photo and add it to the database. Return that group id with each subsequent response.