Synchronise Lookup groups

This causes Lookup groups and their members to be synchronised with
groups in Google. As for people, there are various configuration
options to control which groups to synchronise, and enforce limits on
the number of changes the script will perform.

There is some room for bike-shedding the group name and email, noting
that the email is the group's unique identifier in Google. The current
choices are as follows:

email = {groupID}.groups.lookup@{domain}
name = {groupName}@groups.lookup.cam.ac.uk

The rationale for this is that it will allow for easier searching by
Lookup groupID and groupName (assuming searches operate from the start
of these fields), while maintaining a clear namespace to indicate the
source of these groups, and leaving an obvious namespace choice for
Lookup institutions (insts.lookup), if we import those later.

Note: The main sync.py script has become quite long, and could
probably use refactoring, but keeping it as it is for now to make it
easier to diff.

Closes #10