Re-style the image upload modal dialog. (#356)

Drop the browser-default file input box in favor of our own custom look
and feel, and just tie our button to the file input, and have our own
file name label updated when the file input changes.  Make sure the file
name display has plenty of space for long filenames.
Restyle upload modal for new sightings page (#356)
Abstract out upload modal javascript module (#356)