[Super Editor] - Search by name in stable tag reaction

[matthew-carroll]

One way that stable tags are used in Super Editor is to tag other users. While composing a tag, its desirable to search available users and to try to match an existing user. When searching, it's useful to search by first name and last name with partial values, e.g., "@ma c" to find "Matt Carroll".

To the extent that Super Editor is missing capabilities needed for such a search, add those required behaviors.

This ticket is blocked on #1986