Not possible to create site for clone group after group creation

[boonebgorges]

Reported at https://cboxopenlab.org/groups/help-forum/forum/topic/cloned-groups-and-sites/

During the clone process, we don't show the 'Create a new site' or 'Use an existing site' options. See #356.

This same change applies when you're managing an existing group which was the result of a clone. It can be reproduced as follows:

1. Group A has a site
2. Group B is created as a clone of A, but with 'Set up a site' unchecked
3. B > Settings > Site. The only available option under 'Set up a site' is 'Use an external site'.

Note that the behavior is different if A does not have a site. In that case, when you clone A to B, and you then visit B > Settings > Site, you see all options (new, existing ,external) under 'Set up a site?'

So the question is, what do we want to happen here? When we removed options in #356, what were we trying to accomplish, and how does that reasoning carry over to clones after the initial cloning process?