Run git-auto-commit, linter and tests checks on pull requests

[webignition]

Update the on-event definitions for the git-auto-commit, linter and tests workflows.

• run workflows on all pull requests
  The relevant workflows run only on push events. Triggering these workflows on pull_request event ensures that checks will run when pull request are opened. This allows contributions from forks to have checks run.
• restrict on-push events to only pushes into master
  Currently these workflows are triggered by push events to any branch. Since changes should only be introduced to master via pull requests, triggering these workflows on all push events seems unnecessary.

Summary:

• workflow runs on pull requests ensure checks are run before code reaches master
• workflow runs on pushes to master act as a backup to verify that the post-merge push to master didn't introduce any breakage

[stefanzweifel]

Thanks!
Linting currently fails, but that's not a big issue. I assume this is due to the fact, that the PR is coming from a fork and not having the right permissions.

For now, I've reverted the linter to only run on a push event. tests are imho much more important.