Adds load-from-repo functionality to gist services and new embed

[RedBrogdon]

Changes:

• Updates the new embed to look for four new params in its query string: gh_repo, gh_owner, gh_path, and an optional gh_ref.
• Adds a loadGistFromRepo to GistLoader for loading exercises directly from a GitHub repo.
• Removes the old shareAnon methods, since they no longer function.
• Standardizes gist loading on package:http instead of dart:html requests.
• Moves GistStorage into its own file
• Adds tests for GistLoader's new and existing functions.

GistLoader is getting a little big at this point, but is unlikely to grow again in the short term, so I've left it as a single class with three load methods.

I've added a couple new classes to parse the JSON in the metadata file. These could have been done with built_value or json_serializeable. Since the entire file is <100 lines and neither of those packages is in use elsewhere in the project, though, I've chosen to roll my own rather than take on a new dependency and build step.

A field for "exercise mode" (dart, html, flutter) will need to be added to the Gist class. The new playground will also need to examine that field and set its editors up accordingly. This hasn't been done yet, since John is still finishing the new playground.

[RedBrogdon]

CC @theacodes for research purposes and drive-by comments.

[theacodes]

Out of curiosity: why an empty string instead of null?

[RedBrogdon]

Mostly it's a "that's how it was done in the codebase when I arrived" thing.

[johnpryan]

comments are mostly nits- would like to consider using YAML instead of JSON for the metadata but otherwise LGTM

[RedBrogdon]

PTAL.