Adding a "link" CSS class and removing some duplicate CSS rules
Replacing <div id="playbutton"> with <div class="play"> and removing some obsolete CSS
Fix some minor HTML/CSS issues
Minor fix: change the email input to be a true HTML5 email input (rather than a generic text input)
Reinstate previously broken "Loading" spinner

Using a new spinner image (since we're not using the fancy buttons anymore, which is what the previous spinner image was intended for).
Refactor addValidationError slightly to allow passing null for the field
Minor refactoring for getting the play button and changing its state (enabled vs loading)

This doesn't have any functional impact, but will prevent issues later. There are technically two "play" buttons present on the page (though only one is actually visible), so this.$playButton was actually a set of two elements, instead of just one.
Show an error message in the login form when attempting to log in with an incorrect password (fixes issue #53)