Fix behat tests for insert-an-image.feature

- Fix “I can insert an image from the CMS file store”.
- Remove unused scenario (we no longer support adding multiple images at once from the CMS).
- Fix “I can edit properties of an image before inserting it” scenario.
- Remove “I can edit dimensions of an image before inserting it” scenario - these fields no longer exist.
- Add documentation to explain why this scenario is marked @todo.
Merge pull request #920 from madmatt/pulls/behat-image-uploads

Fix behat tests for insert-an-image.feature