flickr: make use of protocol-relative URIs on the returned source data.

Also, for lightbox, make sure that it uses fully relative paths for
the stylesheets and lightbox as otherwise it can also escape the HTTPS
protocol.
comments: use relative URLs for comment forms.

Using full, absoltue URLs will cause testing environment to comment on
production instances, and will also break HTTPS support on
mixed-protocols sites.
Remove executable permissions to files that shouldn't be executed.
Fix links to relative paths instead of absolute paths.

This unbreaks the view on a local test system and supports HTTPS more
properly.
excerpts: pre-process the excerpts with the filter

This allows posts with markup early in the body to not show said markup.