Add groups resource

[biggujo]

Each group has tasks and users

Important notes:

• if a user is not in /groups/:id/users, reject access to /groups/:id/tasks;
• if a user is not in groups/:id/users, reject access to this route to the user;
• other users cannot join other users (a user only can join/leave the group if he and only he wants to)

Model:

• name of the group;
• array of Users model;
• array of tasks like in Tasks controller and Tasks model;
• timestamps (created_at, updated_at)

Routes:

• GET /groups – get all groups (array of groups);
• POST /groups – create new group;
• GET /groups/:id – get one group:

Schema of response of GET /groups/:id:

{
  "name": "string",
  "members": 123
}

• PUT /groups/:id – update group

Accepted data to PUT /groups/:id:

{
"name": "string",
}

• POST /groups/:groupid/users – add new user to the group (user joins). Works only if userid == current_user.id;

Accepted data to POST /groups/:groupid/users:

{
"id": 12345,
}

• DELETE /groups/:groupid/users/:userid – remove a user from the group (user leaves). Works only if userid == current_user.id;

• ALL /groups/:id/tasks – GET, POST, PUT like in tasks controller etc.