Syncs uploaded posters, improves translation, fixes bugs

• Adds missing translations
• Adds a synchronization option to adopt downloaded posters
• Fixes video player too large on small devices
• Fixes a bug leading to the incomplete deletion of thumbnails
• Fixes non-detection of failing FFmpeg calls
• Fixes FFmpeg availability test failing when using a custom path
• Fixes VideoJS v4 player options not displayed
• VideoJS options only apply to v4 and v5 players for the time being