Shimmie 2.9.1

Fixing the botched 2.9.0 release

• Replace “Image” with “Post” in all user-facing places
• Add PHP 8.0 support
• Keep PHP 7.4 support
• Drop PHP 7.3 support
• Add support for more audio / video formats
• Drop support for Flash
• First-class support for MIME types, preferred to filename extensions
• Use native database Boolean data types
• A ton of smaller bits