T1: Add cover_text text field to post table for phone image layout cover text
T2: Rewrite phone-image-templates.css - remove tpl-magazine/tpl-mixed, merge tpl-minimal into base styles, add content-flow/break-inserter/page-break-marker/page-alignment-toggle styles
- Add content_type column to ul_post via migration
- Install league/commonmark for markdown->HTML conversion
- Add Post model accessors/setters for content_type and content
- Update API Articles controller save/update with content_type support
- Update API docs with content_type parameter and markdown example
Closes content-type-support plan