enable attributions to export/import via WXR

[xolotl]

Currently, Lumen uses an export/import process to bring selections from one Candela course into another. The process works great, but we've recently discovered (been reminded?) that the crucial attributions metadata on each page does not travel with these exports.

The only known workaround is to use export/import to move Candela parts/pages, then manually copy every attribution on every page. Unsurprisingly, this is a very labor-intensive workaround and a number of our highest enrollment and most popular courses (eg, all the English comp courses, college success), require a lot of this type of remixing to tailor them for specific faculty/institutions.

We are also not clear on the differences between WP and PB WXR exports and whether one or the other should be preferred.