Feature/add is migration field to metadata endpoint

[aryan-p03]

What

• Add is_migration field to GET /metadata endpoint (ticket here)
• Bugfix to return the correct website_version link for GET /metadata endpoint (ticket here)
• Bugfix to set the correct URI for the search content updated Kafka event (ticket here)

How to review

For tickets 1 and 2:

• Create a publish a dataset version with is_migration set to true at both series and version level
• Call GET /metadata in publishing mode to see is_migration returned and call in web mode to see it not returned
• Check that the website_version link returned is correct (contains the topic slug). If the dataset is not static then it return it without the topic slug.

For ticket 3:

• Create and publish a dataset version
• After calling PUT /state to transition to published, check the dataset-api logs and within the search_content_updated logData you should see the URI set including the topic slug.

Note:
Technically legacy behaviour has been changed for GET /metadata within publishing mode as URL rewriting has been enabled for the website_version link. The change being that the host on the website_version link when called within publishing mode will be the publishing host (previously this would have been the public host). This is not expected to cause any issues and does affect web mode.

Who can review

• Anyone