Fix confusing white space in membership-new.js

bad indenting makes some of the code look like it's part of a block that
doesn't exist
don't set post on organisation if we've not selected one

If we set the post and one hasn't been selected then we end up with the
UI saying undefined where the post should be when we save it as we have
an empty object so the if ( post_id ) check passed but then it has no
properties

Fixes #448
Avoid displaying undefined in post select of membership dialog

If there isn't a post associated with a membership then don't try and
assign and fetch it when creating the memberships dialog. If you do then
it fetches and assigns all posts and then the code to select the
membership for the dropdown gets confused and displays nonsense.