This adds an embedder for all Spotify resources,
along with a new test page.
In order to make smaller embeds look right,
it also modifies the base embedder to calculate
custom ratios for Bootstrap.

It also refactors error messages for missing IDs
so they can be handled in the parent LinkEmbed class
instead of being duplicated in every file.

In its place, all subclasses need to override two new
class variables: self.site, and self.resource.