Only read text files that look like entries when opening folder journal

[micahellison]

Fixes #1692.

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.

I didn't add a new test for this specifically, but rather, I modified the test folder journal to include entries that should be ignored. Without the change in FolderJournal.py, the tests blow up quite fantastically.

[micahellison]

Talking with @wren about this and I now want to re-do this whole PR:

• instead of using os.walk, explicitly walking through only the folders we want (4-digit year folders on top level, month-like folders on second level; maybe generated a list of months for those)
• using pathlib instead of os.path for comparisons
• add unit tests

[wren]

🕥