clean up URL code, dedupe between web app and browser extension
Created by: sqs
See individual commits.
- deprecate parseRepoURI
- dedupe toAbsoluteBlobURL
- dedupe parseHash, add parseGitHubHash with correct GitHub-specific behavior
- deduplicate repo URL generation code from client/browser
- remove old $references:external URL generation code
There is no intended user-facing behavior change; this is just code cleanup.