Add Phabricator bug importer

[jayvdb]

Wikimedia migrated from Bugzilla to Phabricator in late 2014. Many other large projects have adopted it, or are testing it.

Which I believe means that all projects based on https://openhatch.org/customs/edit/bugzilla/102/MediaWiki are very out of date.

It would be good to switch over to a Phabricator bug importer.

[jayvdb]

Added a task on the other side also: https://phabricator.wikimedia.org/T146969