Transcripts 3rd party integration

3Play Media translation process improvements

EDUCATOR-1468: Includes the following:
- Translation retrieval process do not use the ranslation endpoint which was previously changed by 3PlayMedia
- Make a single call to retrieve all the translation metadata for a video
- Add 100% test coverage for the modified patch

- Fix - HEAL overwrite transcript statuses for completed videos in VAL.
- Add source_language and transcript_status to VideoSerializer and Admin.
- Add tests coverage for Model Serializers.

EDUCATOR-1431 - HEAL process was overwriting transcript statuses as they were combined with transcode statuses. So, separated them out to avoid unnecassary masking of Video's transcript statuses.

token should not use - OR any other character as per regex in url.

File "/home/ec2-user/edx-video-pipeline/VEDA/django_secrets.py", line 24, in <module>
:     DJANGO_DEBUG = return_dict['debug']
: KeyError: 'debug'

- improve logging
- VEDA vedio status updation
- Remove celery task for retriving 3Play translations, accomodate it in heal process
- improve __unicode__ method and add help text
- Add an ability to override developer configs

This implements 3PlayMedia API integration in pipeline.
Other than that it also iuncludes:
-- rename 'Transcript Preferences' to 'Transcript Credentials'.
-- Add migrations.
-- Rename 'Transcription Ready' to 'Transcript Ready'.

See https://github.com/edx/credentials/pull/284/files and
https://github.com/carltongibson/django-filter/issues/562

Yro/update celery queues

Implement global timeout

Fix drf and django filter versions.

Issue: ImportError: Could not import 'rest_framework.filters.DjangoFilterBackend'

DRF-3.7.0 has removed DjangoFilterBackend, if we want to move to DRF-3.7.0, we shall be using 'django_filters.rest_framework.DjangoFilterBackend' instead.