Fix validation errors.

Note that `flex-embed` has its width and height set in the CSS so this wasn't needed anyway.
Update all dependencies to their latest versions.
Remove grunt-contrib-uglify; it's not used anymore.
Remove obsolete JSHint options.
Remove extra commas.
Update jQuery to v1.11.0.
Losslessly compress images.
Add images to filerev.
Switch to the protocol relative URL for the font.
Fix typos in comments and remove unneeded global.
Add `grunt-html-validation` plugin.

Also add a test target which runs the needed tasks.