More roles and permissions (Custom roles?)

Hey,

The feature I am requesting might help quite a lot of people. It is to make a permission-system more advanced than the current one. I was thinking of something where we could do custom roles and give them:
- A Name
- A description (optional)
- A set of permission.

I had a couple of ideas for the permissions. These are only an overview but I think they cover quite a lot:
```
- Admin permissions
- Add Monitors
- Remove monitors
- Edit monitors
- Create Pagespeed
- Delete any Pagespeed
- Delete a set of pagespeed monitors
- Create status pages
- Edit all status pages
- Edit a specific (Or multiple) status pages
- Delete status pages
- Create Maintenances
- Create Maintenances for a specific set of monitors
- Delete any Maintenances
- Delete Maintenances of specific set of monitors
- Create an infrastructure monitor
- Delete any infrastructure monitors
- Delete a set of infrastructure monitors
- Create notification channels
- Delete any notification channel
- Delete own notification channels
- View logs
- Change settings
- Manage Users
- Manage Roles
```

This is, in my opinion, a needed feature and it deserves a place in this wonderful tool that is checkmate.

Thank you