Update experts API endpoint to contain image URLs in featured_media field

[matt-bernhardt]

** Why are these changes being introduced:

• The experts endpoint in the API has a featured_media field, which by default is returned as the ID of the image in that field. However, the legacy platform has overridden this value to be the URL for the image instead, so that the homepage doesn't need to perform a second call in order to load the image.

** Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/lm-304

** How does this address that need:

• This brings forward the api_get_image function from the legacy parent theme, which replaces the image ID with just the URL to the image size we need (for the experts endpoint only)

** Document any side effects to this change:

• The behavior of the experts endpoint is now different from others, like the pages or posts endpoint. The featured_media field is a URL in one, but still the image ID in others.

Developer

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