Pipeline list setting should be a dropdown using pipeline info from Mailman Core

Original LP Author: Terri , LP Link: https://bugs.launchpad.net/bugs/1157875

List administrators have the option to choose the posting pipeline right now, but it's currently a text field. It would be much more convenient (and less deadly) if that could be a drop-down showing the available pipelines. We'll need to be able to get the list of valid pipelines from rest, which isn't possible right now.

It's also possible that we should remove this option entirely and make it a site-admin option, but I can see that people might want to choose between a pipeline with and without anti-spam filters, so we should probably keep it in the web interface.