FR: hyperlinks as episode assets

Continuation of GitHub issue #635.

If the main reference of an episode is web resource, it would sometimes be helpful to have asset types that do not require files to be present in the Media Upload Location. Such as Google Slides, Prezis or journal articles, and have them show up in the same prominent way as the media downloads currently do: colorful button and icon that implies hyperlink & pre-defined label.

Also: YouTube videos was a wish in the ppw15a YouTube session.

PS: Since Auphonic knows the YouTube-URL, it would be cool to be able to import that into Podlove, so it can be accessed via a template & for example auto-inserted as a video embed.