Render lint results as Harbormaster lint messages

Summary:
Ref T8095. Render lint results in a future-ready way.

This makes the renderer accept HarbormasterBuildLintMessage objects. If we have legacy data instead, it converts it into HarbormasterBuildLintMessage objects.

Design is a bit rough but will be cleaned up later after T7739.

This moves away from "postponed linters", which are obsolete after Harbormaster (and were only ever used by Facebook).

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8095

Differential Revision: https://secure.phabricator.com/D13377