Minor changes to Image field (for consistency)

Inconsistencies in image field implementation found while creating the design for link fields (https://issues.onehippo.com/browse/CHANNELMGR-1399)

if field had focus:

• line under field should become thicker and blue.
• image itself should be skipped in tab order

if field is empty:

• the select button should be positioned on the far right
• the label should remain #000000 54% if the field has focus

See attached design image