WIP: SE-2589 fix AttributeError: 'LibraryLocatorV2' object has no attribute 'make_asset_key'

Description

Previously, getting transcripts for a video stored in blockstore through the xblock rest api was broken:

```
File "/edx/app/edxapp/edx-platform/openedx/core/djangoapps/xblock/rest_api/views.py", line 44, in block_metadata
metadata_dict = get_block_metadata(block, includes=includes)
File "/edx/app/edxapp/edx-platform/openedx/core/djangoapps/xblock/api.py", line 105, in get_block_metadata
data["index_dictionary"] = block.index_dictionary()
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/video_module.py", line 1050, in index_dictionary
_update_transcript_for_index(language)
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/video_module.py", line 1037, in _update_transcript_for_index
transcripts, transcript_format='txt', lang=language
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/transcripts_utils.py", line 816, in get_transcript
data = Transcript.asset(self.location, transcript_name, lang).data.decode('utf-8')
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/transcripts_utils.py", line 702, in asset
return Transcript.get_asset(location, asset_filename)
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/transcripts_utils.py", line 709, in get_asset
return contentstore().find(Transcript.asset_location(location, filename))
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/transcripts_utils.py", line 719, in asset_location
return StaticContent.compute_location(location.course_key, filename)
File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/contentstore/content.py", line 84, in compute_location
return course_key.make_asset_key(
AttributeError: 'LibraryLocatorV2' object has no attribute 'make_asset_key'
```

There were two `get_transcript` methods. The broken one that is being used here (`VideoTranscriptsMixin.get_transcript`) is stripped out in this PR - it has been superseded by `transcripts_utils.get_transcript`. The latter includes support for blockstore and VAL, while the former did not.

*JIRA tickets*:

*Dependencies*: None

*Sandbox URL*: TBD - sandbox is being provisioned.

*Merge deadline*: None

*Testing instructions*:

1. TBD

*Reviewers*

  • [ ] @bradenmacdonald

  • [ ] edX reviewer[s] TBD

*Settings*
```yaml
```

Assignee

David Ormsbee

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Samuel Walladge

Repo

edx/edx-platform

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

Priority

Unset
Configure