Allow admins to set membership type for group type

This MR brings over the changes from github for #333. I've tested it locally and it seems to be working.

A select field is added on the group type form, where you can select the membership type to set as the default for the group type.
Requesting a membership for that group then creates a membership of the defined type.

The current development branch is 2.x. Let's move the work there. We will evaluate later if this needs backport to 1.x