Shrink reprs

[aazuspan]

Shorten CSS class names and remove unused classes to shrink HTML repr size as much as practical. Based on the benchmark below, I was able to drop repr size by about 30%!

obj = ee.ImageCollection("COPERNICUS/S2_SR").limit(1000)
rep = obj._repr_html_()
mb = len(rep) / 1e6
mb 
# Before changes: 68.84692
# After changes: 48.22587

I could have dropped another ~3% off the file size by setting type='checkbox' on the <input> elements within the JS instead of the HTML, but that seemed too weird, and I'm hoping to drop the JS script and go pure CSS in the future (pending new selector support, see #5).

Close #11