Use full resolution on small images in edition forms

Closes #707 (closed)

This removes a confusing anti-pattern with the widen property on ElementImage and updates the images displayed in the element creation, element edition and transcription edition modals. The images now use the full resolution when width+height < 1400.

widen was used "in reverse" just to have ElementImage do a check for the polygon being under 500 pixels only for the transcriptions modal; the property was sync'd and never set by the parent modal or used in the child ElementImage; it was instead set by the child and used by the parent. This was just carried over to the modal.

While testing this, I found two more bugs; #716 (closed) can cause some issues here if the polygon is very small or the user's browser window very large since the image would not display at all, and #717 (closed) is not a major problem but could cause some confusion if rather precise polygons are necessary.