Don't load Pins "in background" when lightbox is active.

Remove the scroll event handler when lightbox is active, so pins are
not loaded in background breaking the page layout.
Generate thumbnails when image is being saved

Too lax unique constraints for Thumbnail coped with "on demand" thumbnail
generation may leave database in inconsistent state where two thumbnail for
the same size are saved. We should be able to prevent that from happening
by generating all thumbnails when we save the image. Should fix #24, but
I can't figure out a way to actually test it.
Pin factory-boy dependency on 1.3.x branch

Factory Boy 2.0 have removed the "extracted_key" keyword argument from
post_generation() hook. Until we can update tests to work with 2.0 make
dependency on 1.3.x branch. In turn, the update got stalled by a bug
in Factory Boy (https://github.com/rbarrois/factory_boy/issues/53).
Update version to 1.1.0 in preparation for the release
Merge branch 'release/1.1.0'