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.

For the full list of versions to be displayed, [this change on cdn.ariatemplates.com](https://github.com/ariatemplates/cdn.ariatemplates.com/commit/7306c43fcfeea32995ad277e7323642328d7b0f3) still has to be deployed in production. However, if it is not deployed yet, there is no error, and the list only contains the current version.