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).