VE-782: add loading overlay when the editor is loading.
VE-782: Remove commented logic and leave note about why
VE-782: move loading indicator so it always displays, not just on initial load.
VE-782: animation cleanup and minor refactoring

Now only uses JavaScript animations. Couldn't get reliable animations in
CSS, probably because of adding/removing from DOM. Also make
loadingIndicator method a bit more generic so it can be used with other
kinds of indicators in the future (like saving).
VE-782: fix typo.
VE-782: removing unnecessary import
Merge pull request #2724 from Wikia/VE-782

VE-782: add loading overlay when the editor is loading.