Shimmie 2.8.0

Potentially compatibility-breaking changes:

• PHP strict_types enabled
• themes/<theme>/layout.class.php merged into themes/<theme>/page.class.php
• ext/handle_static renamed to ext/static_files
• ext/handle_404 renamed to ext/four_oh_four

Added extensions:

• approval
• auto_tagger
• handle_cbz (beta)

Removed extensions:

• oekaki (java in the browser is deprecated)

Other notable things:

• Much improved Docker support for both testing and deployment
• Use of MicroHTML in many themes
• Use of MicroCRUD for generic tables
• Switch from Travis to GitHub Actions