Handle more weird errors without asploding
Make reading mail on the CLI work again
Improved CLI search results, made CLI message threads navigable
Fixed broken URL mangler, made less easily frightened
Added attachment_type jinja extension for #654
Made composer show up in Reply thread, even if already replied to a message
 Enhanced plugin loading for external plugins.

 New manifest.json features are:
 - startup and shutdown hooks
 - register mailboxes
 - pass config to mailbox parse_path
 - Added usage examples to demos plugin

 Necessary enhancements for external LEAP plugin.