Le-utils

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- Added new labelslookup.json constants
- Added labelstemplatelookup.json and idforlabels.py
- Added build-label target for Makefile release

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- Added `zh-Hant` language code and updated tests
- Added Marwari and Bhojpuri langauge codes

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- Added `m4v` to the list of convertible video formats
- Added constants for `h5p` content type

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- Added additional formats for other caption file types that are convertible by `pycaption`:
- DFXP
- SAMI
- SCC
- TTML

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- Fixed `document_thumbnail.order` so previews default to the document instead of the thumbnail

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- Added necessary constants for Slideshows:
- New content kind added: `slideshow`
- New format presets added:
- `slideshow_image`: represents a node-file association of the individual slideshow images part of the slideshow content node
- `slideshow_thumbnail`: the thumbnail for content nodes of kind=slideshow
- `slideshow_manifest` (allowed_formats: `.json`) :
the json manifest file for the slideshow generated during `exportchannel` for Kolibri consumption.
This file will contain json of slideshow data computer from the studio DB
(contents=association info `studio.models.ContentNode` --fk-- `studio.models.SlideshowSlide`)