Consolidate image validation into ImageValidator

Image validation, especially the validation of image size
was repeated all over the place. This adds a validator class
to cover both validating an image is readable, that it's mime-type
matches the file extension and that it is of the correct size.

Removes ImageSizeChecker.
Add test to verify attachment upload whitelisting
Merge pull request #1061 from alphagov/fix-imagemagick-error-messages

https://www.pivotaltracker.com/story/show/44008053

Consolidate image validation into ImageValidator