Move group membership views into their own file

[seanh]

...plus a little tidy up.

I wanted to also tidy them up into a nice view class but I don't think that's going to be possible without first doing a bunch of work to remove h's custom @api_config decorator (Slack thread). Not worth the effort right now, it's fine how it is. But if it wasn't for @api_config it would be possible to do something like this:

@view_config(
    route_name="api.group_members",
    request_method="GET",
    permission=Permission.Group.READ,
)
def list_members(context: GroupContext, _request):
    return [UserJSONPresenter(user).asdict() for user in context.group.members]


@view_defaults(route_name="api.group_members")
class GroupMembershipAPIViews:
    def __init__(self, context: GroupMembershipContext, request: Request):
        self.context = context
        self.group_members_service = request.find_service(name="group_members")

    @view_config(request_method="DELETE", permission=Permission.Group.MEMBER_REMOVE)
    def delete(self):
        self.group_members_service.member_leave(context.group, context.user.userid)
        return HTTPNoContent()

    @view_config(request_method="POST", permission=Permission.Group.MEMBER_ADD)
    def post(self):
        if context.user.authority != context.group.authority:
            raise HTTPNotFound()

        self.group_members_service.member_join(context.group, context.user.userid)
        return HTTPNoContent()

    @view_config(request_method="PATCH", permission=Permission.Group.MEMBER_EDIT)
    def patch(self):
        ...

    @view_config(request_method="GET", permission=Permission.Group.READ)
    def get(self):
        ...