This change adds a GitHub App as an alternative to Personal Access

Tokens. This involves a bunch of moving parts:


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.


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.


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.

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.