Support rebasing PR from forks and allow only selected users to rebase (

cirrus-actions#17)

* Support rebasing PR from forks

This works only if the "Allow edit from maintainers" option is checked
and the `GITHUB_TOKEN` variable is set using a developer token, i.e.

env:
  GITHUB_TOKEN: ${{ secrets.GITHUB_OAUTH_TOKEN }}

* Use the user name and email for committer name

Instead of using a generic name and address, fetch these information
from using the GitHub API.

* Allow user based tokens

If an environment variable `<user_login>_TOKEN` exists, that variable is
used instead of `GITHUB_TOKEN`.

This makes it possible to allow only some users to rebase the PRs by
setting

  env:
    <user1>_TOKEN: ${{ secrets.<user1>_TOKEN }}
    <user2>_TOKEN: ${{ secrets.<user2>_TOKEN }}

Exit with code 0 when nothing to do

Workaround for cirrus-actions#14

Fixed workflow location

Rename action to Automatic Rebase