* Add updated methods to get teams and team membership for a user.

* Fix formatting

* Add tests and cleanup the orgs service.

* Formatting.

* Update formatting.

* Cleanup error codes.

* Formatting.

This is primarily based on the timeline API response schema (using code generated from the schema by the script in `tool/process_github_schema.dart`). Where I could find existing model classes compatible with bits of that schema I tried to re-use them, extending them as necessary. I took some liberties where the GitHub API documentation was vague, contradictory, or otherwise dubious.

The comments on the new code are based on the comments in the response schema. They're not super helpful, if I'm honest.

I found that some of the model classes were inconsistent in various ways and tried to clean up the code near what I touched to be more consistent. For example, I made a bunch of fields on a bunch of model classes mutable where previously they were `final`, to be more consistent with other model classes. I also moved some constructors to be above their corresponding fields to be more consistent with other code that already did that.