Feature: Preview uploaded files in the editor

[lindapaiste]

Fixes #57

Changes:

• Creates an AssetPreview component which is rendered inside the editor container when the user clicks on a file with a url (that is, an asset uploaded to S3).
• Gets the mime type from the file name using package mime (which we use on the server).
• Handles the following mime types:
  • image/* => <img> element
  • video/* => <video> element
  • audio/* => <audio> element

I might need to tweak the CSS for images which are taller than they are wide. I'm using max-width: 100%; height: auto; for both image and video.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

This is really neat, thanks so much for working on this!