TODO
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* post filters
* fts
* projects
* readonly-db connection (using `PRAGMA query_only`)
* per-site public/ directories
* history (in `PostModel#update_post`)
* triggers for removing unused `post_tag` and `tag` entries

Editors
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* tinymce (http://tinymce.com/)
* ace (https://ace.c9.io/)
* codemirror (http://codemirror.net/)