Get content with groups

Currently, the backend doesn't return the group(s) a content is in.

mentioned in issue #29 (closed)

I think there are two possible solutions:

1. Emit group(s) whenever a content is queried in the backend
2. Only allow the group to be set with the content on content creation. Add different route to change the groups a content is in

@dgrh99 personally, i'm a fan of the first solution.

mentioned in commit be571bb7

mentioned in merge request !106 (merged)

assigned to @dgrh99

closed via commit be571bb7

mentioned in commit 6987618d

closed via merge request !106 (merged)