Rename TypehaeadUserParameterizedDatasource to PeopleUserFunctionDatasource

Summary: Ref T4100. This improves application scoping (Typeahead -> People) and uses a more consitent and concise name (Parameterized -> Function).

Test Plan:

• grep
• Used affected datasource.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D12532