Allow users to manage their own groups

[psrok1]

Current permission model involves groups that allow users to share samples and all related artifacts within one of their own groups. That's how public feed works - each user is member of "public" group, so everybody can share some objects with all users.

This feature can be used to create groups for organizations that want to have common workspace in Malwarecage. Unfortunately, all group management features and knowledge about group members can be accessed only by Malwarecage administrator (needs manage_users capability) which limits the feature usability for external users.

Proposed improvements:

• Show list of members of own groups in 'Profile' view, excluding private and public groups
• Allow to invite other users to own groups using invitation link
• What with creating new workspaces? Request to admin? We don't have yet any feature that allows to send Action required notification via Malwarecage. Maybe we should optionally allow users to register custom workspaces?
• What with removing users from workspaces? (e.g. user is no longer employeed in company related with group)

Moved from CERT.pl internal repository. Reported originally by psrok1