Store images in multiple sizes. Preserve original

[docapotamus]

At the moment all images are resized to 1280 x 720 maximum.

If they are smaller that is fine but any larger they will lose something.

I'm proposing storing the image in it's original size, and making a couple of smaller thumbnails to use around the site.

This would help with data usage on the main site and load times.

We can store the image URL's along with the post as we do now.

This work could be done whilst looking at #241

[docapotamus]

From an engineering point of view may be worth waiting for block storage at Digital Ocean.

We store images in Mongos GridFS but we eventually want to move to PostgreSQL so using a file system would make more sense.

@mcrmonkey

[docapotamus]

#338