Update copyright notices for 2023

[wren]

This updates the copyright notices on all the files to 2023 (from 2022):

Copyright © 2012-2023 jrnl contributors
License: https://www.gnu.org/licenses/gpl-3.0.html

It also standardizes the whitespace after the copyright notices to include one blank line (most files did this already, but a few did not).

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• 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.

[micahellison]

📅