Lab head role

We have labs, with corresponding Django groups to handle permissions on classifications.

There is a LabHead model, but I don't think it's used.

There is a parallel implementation of "lab heads" by assigning them to a group of "lab_name/head", which is used in classifications. There is some code that uses a ```LabUser``` class (not Django model) eg like:

```
{% for lab_user in lab.lab_users %}
{% if lab_user.role == 'head' or is_member %}
```

It would be good to consolidate the single lab head implementation, and then make a bunch of convenience methods

The goal would be to allow lab heads to have more authority, eg like superusers for some tasks (eg modify restricted things on classifications, or assign users to a lab etc)