Adhere to XDG spec for file locations

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

Use different directories for different purposes:

• dirs::config_dir for config file, with a check to use the old dirs::data_dir if present for backward compatibility
• dirs::data_dir for DB (same as current location)
• dirs::state_dir for logs, falling back to data::cache_dir on MacOS/Windows

Closes #371

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

• Could be a breaking change. Mitigated with a backward compatibility check
• Added complexity. Mitigated with docs

QA

How did you test this?

Manually.

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked