Add settings directory and yt-dlp metadata import feature

[rogermparent]

• Add "settings" directory, which is effectively a second content directory which holds site config that may not need to be synced between instances
• Use yt-dlp -P to scrape YouTube video metadata and pre-fill the import form with it
• Define yt-dlp bin location with settings
• Test setting yt-dlp bin
• Test yt-dlp fetching by using a dummy binary with fixtures
• Change content directory behavior to default to test-content when a test mode env var is set instead of specifically setting CONTENT_DIRECTORY=test-content on each test script, as now settings needs the same treatment