local coauthors was messing with unsupported post types

The query was being altered by local coauthors plus even if the post type being queried was explicitly unsupported for coauthors.

This resolves a compatibility issues between `go-alerts` and coauthors-plus.  The issue was reported here: https://github.com/GigaOM/legacy-pro/issues/3088
Merge pull request #21 from zbtirrell/better-post-type-exclusions

local coauthors was messing with unsupported post types