Added the libvips ruby gem.

Added a model to handle stories images, fetching the image from the
original post that the story is linked to and adds a Lobsters logo with
at most 1/5 of the image height in the lower-left corner of the image.

Added a job to fetch the story image card async.

Following the same idea on how avatar are handled on lobsters, now the
image from the original post that the story links to are also cached and
saved in a local folder.

The og:image link should point to its story image.

Ignored the FileAccess warning, as `File.basename` sanitization ensures
that no relative path will be injected into the request.