feat(github): Use Github App

[violeine]

Here something that memos.pub benefits from github app:

• github app use its own api token

  A GitHub App acts on its own behalf, taking actions via the API directly using its own identity, which means you don't need to maintain a bot or service account as a separate user.

• repo configuration become easy with single file permission

• invalidated cached with webhook (github app can subscribe to repo's event)

• increase visibility through github marketplace

• still "without setup or sign up"
  since user's content already hosted on github, installing a github app is as easy as enabling github page

links: about apps, permission, example, ko-fi