Add timelens support for Relive recordings. This works similarly to the current scrub thumbs, but is mutually exclusive with it as both features overlap technologically as well as from a user perspective.

Currently, the Timelens CSS gets added dynamically because loading it indiscriminately results in artifacts in the player. Not sure if we either need a portable way to do this in JS (h() cannot be used, as it's a PHP function and templating the JS with PHP doesn't seem to be a sane solution) or if this can be fixed on the Timelens side. Comments welcome.