Fix Unicode encoding failure in directory export when creating filenames from journal titles with certain characters

[micahellison]

Fixes #1089.

It turns out that strftime doesn't play nicely with all Unicode characters. I think this is a Windows-specific issue, but I'm not sure, and we're testing on all platforms anyway. This PR modifies the directory export's file name builder to use strftime to handle only the date part of the filename, then append the rest of the string, which may contain just about any character.

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]

🎐

[github-actions]

Unit Test Results

  16 files  ±0    16 suites  ±0   9s ⏱️ -1s
278 tests  - 6  278 ✔️  - 6  0 💤 ±0  0 ❌ ±0 
279 runs   - 6  279 ✔️  - 6  0 💤 ±0  0 ❌ ±0 

results for commit 259a607 ± comparison against base commit 046ebc7