Merge tags on upload, style changes, etc.

When you upload a screenshot, if it existed before and you add tags in
your submission, we will now merge the tags with the existing tag list
so they aren't lost.

The screenshots now have a little box-shadow. If a screenshot was
assigned to the top-level object it will have a black box-shadow (also
helps with differentiating white in the image with the background of the
UI). If a screenshot was discovered through the bucket list tags, it
will show up with a blue box-shadow.

The Screenshots section now collapses properly.
Screenshot API and other fixes.

Added Screenshots to the API.

Had to make some changes to the core API to support images being added
to the zip files that it outputs so you can do ?file=1 on a screenshot
and get the image back.

You can read Screenshots and upload screenshots through the API. There
are two modes, "ids" and "screenshot". The former asumes you are
uploading a comma-separated list of ObjectIds of existing screenshots to
append to a top-level object. The latter assumes you are uploading an
image to the system.

Fixed some bugs with uploading screenshots with no description, no tags,
or no source information.
Help documentation for the screenshot API.