Shimmie 2.7.0

Potentially compatibility-breaking changes:

• Imageboard-specific core code moved to core/imageboard
• Lots of custom bulk-action tools replaced with a unified Bulk Actions extension
• lib/static moved to ext/handle_static
• Extension metadata moved from main.php to info.php

New extensions:

• media
• transcode
• trash
• user_config
• post_titles
• help_pages
• relationships

Removed extensions:

• chatbox (full of security issues)
• amazon_s3 (never really worked properly)