Change tree photos UI to show multiple photos

Changes the treemap endpoint to return an HTML fragment, used to create a
Bootstrap Carousel.

Modifies imageUploadPanel.js to be able to handle either an endpoint that
returns json with a URL or errors, or an endpoint that returns HTML
already containing relevant error messages.
Extend the carousel on the tree detail page to have scrolling thumbnails

Bootstrap Carousel has support for 'indicators', but they are intended to be
navigation pills overlaid on the image.  We are making use of the navigation
control built into the indicators, but making them slide back and forth under
the image in the style of Flickr's image thumbnails.