Initial 'garbage collection' of unused cached images when cleaning history

Images linked from sections were saved into the database, but not deleted
automatically when their section was removed because they might be used
in multiple articles.

Adds a convenience method [image deleteIfUnused] to kill out the old images
if still unused after the cascading deletes remove the sections.

Change-Id: I6755ec0490e16d1d46fbb34e61c4d947858989a1
Merge "Initial 'garbage collection' of unused cached images when cleaning history"