Add RawGithubReadInstance wrapper for improved UX

[blueridger]

Description

Finding and composing a raw github repo url is confusing, and we get tech support questions about it regularly. This adds a more user-friendly factory for instances that use raw github fetching.

Test Plan

yarn build && yarn shell

$ i = sc.instance.readInstance.getRawGithubReadInstance("sourcecred","cred","gh-pages")
Vd {
  _storage: Mu {
    _base: 'https://raw.githubusercontent.com/sourcecred/cred/gh-pages/'
  },
  _zipStorage: Su {
    _baseStorage: Mu {
      _base: 'https://raw.githubusercontent.com/sourcecred/cred/gh-pages/'
    }
  }
}
$ i.readLedger().then(l => console.log(l))