This change adds a GitHub App as an alternative to Personal Access Tokens. This involves a bunch of moving parts: 1. The actual GitHub App backend, which includes a redirect view that will trigger the GitHub App Manifest flow, and callback views for when the app has been created and when the app has been installed into a user or organization. This ends up creating a hidden `HostingServiceAccount` which holds the app registration keys, and then each time the app gets installed (for a user or organization) will create a new `HostingServiceAccount` for that entry. 2. Modifications to the GitHub client to use the app for authentication. This involves minting short-lived RS256 JWT tokens for the actual API requests to GitHub. 3. Customized UI parts for the connected services list. This involves: - Special templates and flows for the "Connect a service" wizard which lets users choose how they want to connect, and triggers for the new app creation flow. - An override for the entry template which adds an extra controls panel that either encourages users to create the app (if they have existing connections using PATs only), or provides a link into GitHub's settings UI (allowing changes to URLs or transferring the app registration to another owner). This also currently includes a stub webhook handler (since GitHub Apps have a single webhook endpoint for all authorized repositories), as well as some commented-out bits for future enhancement. Testing Done: - Connected a GitHub App and installed it into multiple organizations. - Tested all aspects of the wizard flow, including creation of the app and reassignment of any PAT-based repositories after app install. - Verified that API access works correctly when authenticating using the app tokens. - Ran unit tests.