Import PCPPs

Extends the donation amount synchronization script to also account for project imports.

* Adds a `published` field on PCPPs, defaulting to False; data migration to move all existing entries to be published
* Allow featured_image to be blank on projects
* Adds a `published_objects` manager on PCPPs, which is used throughout the site
* When importing project information, look up the associated sectors/country/etc.
* If it can't find the sector/country referenced, it'll log a warning, but continue

Note that we may need to tweak the sector mapping once we have a better dataset.