Rename "QuickSearch" Engine/EngineExtension to "Datasource"

Summary: Ref T13079. This recently-introduced Engine/EngineExtension are a good fit for adding more datasource functions in general, but we didn't think quite big enough in naming them.

Test Plan: Used quick search typeahead, hit applications/users/monograms/symbols/etc.

Maniphest Tasks: T13079

Differential Revision: https://secure.phabricator.com/D19087