Add client filtering of conversation part types for authorized users

[daveconnis]

Problem

Add the ability for authorized users (admin) to filter by conversation part type.

This is currently blocked by 1638 and 1689.

Add client-filtering of parts based on authorizaton

Wrap non-comment parts in the conversation thread under a {{#if canAdminister project}} helper, to hide non-comment parts from users who aren't admins on project.

Add integration tests to support this change

Render a thread with one of each supported part type, register a fake project ability with canAdminister: true
Render a thread with one of each supported part type ,register a fake project ability with canAdminister: false, see if just the comment part is rendered
See tests/integration/components/task-assignment-test.js on how to register a fake ability

References #1636 #1638, #1689

Progress on: #1597