Add active class to links via js instead of directly in l()

Part of #1830598: [meta] support render caching properly.

The active class added by l() means that any content rendered with an l() call in it can only be cached per page - otherwise the .active class will be wrong.

If we added that via JavaScript instead, the markup would be the same for all pages. This applies to the toolbar, menu blocks, and most entity rendering.