Change all YAML FullLoader calls to SafeLoader

[micahellison]

The pyyaml project has some security issues with its FullLoader. It's not a huge deal for jrnl users, given that it's from the config file rather than untrusted input (such as user supplied data to a web server), but since all the tests are passing with SafeLoader, there's no harm in switching to this.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have tested this code locally.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.
• All tests pass.

[wren]

🥔