Add carrierwave_direct gem

So that we can upload directly to S3.
Remove unnecessary comments from uploader
Rename Video#video to #content

To avoid some confusion.
Remove unnecessary comments in routes.rb
Upload videos directly to S3

This changes the flow as we now upload the video to S3 before creating
the `Video` object and storing it in our database.

Once uploaded, we redirect to `Videos#new`, instantiate a new `Video`
object and set the `key` attribute to the key in the params. This `key`
contains the partial path of the new video on S3. We emit this path in
a hidden field in the new video form so that it's stored when we create
the `Video` object in our database.