Improve video integration (VideoBlock, video_config, edx-val, edx-platform)

We created the video_config service in edx-platform as part of the XBlock Extraction. The new service was a quick way of abstracting out the edx-platform-specific parts of VideoBlock, allowing us to extract the VideoBlock to a new repo.

But video_config is not a perfect interface, for a few reasons:

• It takes the entire VideoBlock as a parameter to some methods, and modifies the fields on the VideoBlock, which breaks the abstraction.
• It's more than just config: it's transcripts too.
• It's somewhat redundant/overlapping with edx-val.
• It handles Course videos and Library V2 videos in questionably different ways. For example, edx-val is used for Course transcripts but not for library transcripts. Why's that?

For this ticket, suggest refactorings to VideoBlock, video_config, edx-val, and/or the rest of edx-platform that would make the interface cleaner. Things to keep in mind:

• do we want to invest in edx-val or deprecate it in favor of something else?
• do we want videos to work without being tied to an xblock?
• do we want to start fresh with a new video xblock?

Start by reading the VideoBlock, video_config, and edx-val interfaces and tracing how edx-platform handles Course and Library V2 videos, transcripts, and configuration. Done means documenting and proposing a cleaner boundary among these components, including a decision about edx-val, XBlock coupling, and whether a new video XBlock is needed.