stylesheets: Remove dynamic stylesheet serving.

Since the introduction of the media providers and the default
installation of the filesystem media provider, it's no longer necessary
for local / non-AWS installs to use dynamically served stylesheets.

This patch removes that option to reduce complexity in the stylesheet
flows.
media: Take advantage of HTTP & HTTPS media being on the same domain.

Since we have an HTTPS-capable CDN in front of our S3 static domains
now, it's far faster for clients to use the CDN on HTTPS as well rather
than going straight to (high-latency) S3.

This patch makes it so that we continue to store URLs with explicit HTTP
schemes but instead of conditionally converting to HTTPS, we render
protocol-relative URLs. This should be safe for systems using the
filesystem media provider as we've installed an SSL cert there all
along.
stylesheets: Add protocol-relative stylesheets.

Now that we have protocol-relative media objects, there's no need to
maintain HTTP and HTTPS variants of subreddit stylesheets. Instead,
we'll start writing all new stylesheets with protocol-relative image
URLs. Old subreddits will continue to use their protocol-specific
stylesheets for now until new stylesheets are uploaded or existing ones
migrated.