Add conversation part for open/closed status changed

# Problem

We need to add a conversation part that includes a simple border and some small text saying either:

- {{You | username}} closed this conversation {{time-ago}}
- {{You | username}} reopened this conversation {{time-ago}}

These parts should not be shown in the user's view of a conversation. Only the `comment` `part_type` should be shown. We'll have to add a check for whether the user has admin privileges to see that information.

This is blocked by the API adding a `part_type` for us to determine the type of conversation part to display.

Also needs https://github.com/code-corps/code-corps-api/issues/1322 to be done to be mergeable.