Implement front-to-back GitHub OAuth support on blocks

[Pwuts]

• GitHub blocks (supplied by @Bentlybro ) - Add GitHub blocks #8006
• Block UI elements to choose credentials / initiate authorization - Block UI elements to handle authorization #8005
• Mechanism to authenticate -> create new credentials - Integration OAuth front end mechanism #8004
  • Get login URI from back end
  • Open login URI in pop-up window
  • Detect redirect and extract code from pop-up window (and close it)
  • Exchange code for tokens with back end
• API endpoints to retrieve credentials - API endpoints to retrieve integration credentials #8003
  • list available credentials for a provider (without the secrets)
  • get credentials by ID

In many cases, the front end does not need the credentials themselves, but just needs to know that they are available and some metadata to display identifying information for a specific set of credentials (e.g. login name). In some cases though, the frond end will need the credentials, e.g. for a Google Drive integration to pick a file or folder in the front end.

I have chosen to start with GitHub (despite Google being higher overall priority) because GitHub will be more straightforward and thus will allow us to lay a foundation for this entire end-to-end flow more quickly.