Add link field support for Media entities

Problem/Motivation

The autocomplete form on a Link field does not produce results from Media items. A common scenario is linking to a Media item, such as a document.

Steps to reproduce

1. Configure a Document media type.
2. Add a Document to the site.
3. Add a Link field that allows for internal links.
4. Type the name of the Document added to the site within the Link field.

Proposed resolution

Include Media items in the autocomplete results of a Link field.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet