Call embed-channel after public channel publishing

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Dependent on: https://github.com/learningequality/curriculum-automation/issues/58

Current behavior

We're not currently using the curriculum automation API to embed content from Studio, and so when a public channel is updated, we must manually regenerate embeddings for its content.

Desired behavior

• A new request class EmbedChannelRequest(EmbeddingsRequest) should exist for making POST requests to /embed-channel with a channel_id (we're hardcoding the request schema at this time)
• A new method should be added to RecommendationsAdapter to accept and execute the EmbedChannelRequest
• A new method should be added to AutomationManager to call the above method, accepting a channel_id and creating a EmbedChannelRequest
• The function publish_channel should:
  • call the above method if and only if its a public channel
  • not fail publishing if calling embed-channel fails, but does report the failure to sentry

Value add

Automates bulk regeneration of embeddings for public channels after they're updated, until we've improved performance and implemented tighter API integration using embed-content requests

Possible tradeoffs

We have API methods that we specced to embed content on a node by node basis. Long term, that will be helpful to integrate into Studio's workflows, but will require us to be thoughtful on its impact to the performance of channel publishing. Also, we need to improve the performance of embedding storage to minimize request time.

Start by tracing publish_channel and how it reaches AutomationManager and RecommendationsAdapter; the issue also depends on curriculum-automation issue 58. Verify the existing request classes and publishing flow before adding the embed-channel request path. Done means public-channel publishing triggers the request, failures are reported to Sentry without aborting publication, and non-public channels do not trigger it.