feat: POST-only non-idempotent endpoints

[GrygrFlzr]

• Make sure data directory exists before using it
• Fix favorites always being blank on server start
• Make /favorites, /note, /flag_content endpoints POST-only
• Fix app always writing to disk even before 60 seconds have passed
• Fix app error trying to delete nonexistent query parameters
• Fix redirects after POST action
• Color flag count when greater than 0
• Fix invalid CSS property text-weight -> font-weight
• Fix incorrect query parameter handling on Share Link
• Graceful timeout if update_all fails

Closes the following issues on kagifeedback:

• https://kagifeedback.org/d/3312-small-web-appreciate-and-flag-can-potentially-be-triggered-by-browser-page-preloading
• https://kagifeedback.org/d/3312-small-web-appreciate-and-flag-can-potentially-be-triggered-by-browser-page-preloading
• https://kagifeedback.org/d/3310-small-webs-link-here-does-not-account-for-existing-query-parameters

[vprelovac]

Very nice PR, thank you for contributing. Should be live soon.