Allowing to change the version of Aria Templates

With this PR, the Aria Templates CDN is used instead of the local version of Aria Templates. The version number can be passed in the url and an html select displays the list of available versions.

Displaying the full list of versions relies on [this change on cdn.ariatemplates.com](https://github.com/ariatemplates/cdn.ariatemplates.com/commit/7306c43fcfeea32995ad277e7323642328d7b0f3) which is now deployed in production (cf [here](http://cdn.ariatemplates.com/versions)).