Add client filtering of conversation part types for authorized users

# 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

先查看阻塞項 #1638 和 #1689，然後以 tests/integration/components/task-assignment-test.js 作為註冊 fake ability 的參考。為包含每種受支援 part type 的 thread 新增整合測試覆蓋，並分別測試 canAdminister 為 true 和 false 的情況；完成標準是 admins 能看到所有 parts，而 non-admins 只能看到 comments。

由索引模型根據 Issue 內容生成。