Ricecooker

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* Added support for tags using the ``JsonChef`` workflow
* Added validation step to ensure subtitles file are unique for each language code
* Document new ``SlidesShow`` content kind coming in Kolibri 0.13
* Added docs with detailed instruction for content upload and update workflows
* Bugfixes to file extension logic and improved error handling around subtitles

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* Updated documentation to use top-level headings
* Removed support for Python 3.4
* Removed support for the "sous chef" workflow

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* Handle more subtitle convertible formats: ``SRT``, ``TTML``, ``SCC``, ``DFXP``, and ``SAMI``

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* Updated docs build scripts to make ricecooker docs available on read the docs
* Added ``corrections`` command line script for making bulk edits to content metadata
* Added ``StudioApi`` client to support CRUD (created, read, update, delete) Studio actions
* Added pdf-splitting helper methods (see ``ricecooker/utils/pdf.py``)

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* Updated ``le-utils`` and ``pressurcooker`` dependencies to latest version
* Added support for ePub files (``EPubFile`` s can be added of ``DocumentNode`` s)
* Added tag support
* Changed default value for ``STUDIO_URL`` to ``api.studio.learningequality.org``
* Added ``aggregator`` and ``provider`` fields for content nodes
* Various bugfixes to image processing in exercises
* Changed validation logic to use ``self.filename`` to check file format is in ``self.allowed_formats``
* Added ``is_youtube_subtitle_file_supported_language`` helper function to support importing youtube subs
* Added ``srt2vtt`` subtitles conversion
* Added static assets downloader helper method in ``utils.downloader.download_static_assets``
* Added LineCook chef functions to ``--generate`` CSV from directory structure
* Fixed the always ``randomize=True`` bug
* Docs: general content node metadata guidelines
* Docs: video compression instructions and helper scripts ``convertvideo.bat`` and ``convertvideo.sh``

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* Added support for ``role`` attribute on ConentNodes (currently ``coach`` || ``learner``)
* Update pressurecooker dependency (to catch compression errors)
* Docs improvements, see https://github.com/learningequality/ricecooker/tree/master/docs