Display pull request events in main review discussion

We should display some or all of the events that GitHub shows in the PR conversation, such as commits, review requests / approvals, etc. However, these events shouldn't show up as unreplied for users and shouldn't require acknowledging.

Internal implementation notes:
- We can fetch the events via GraphQL from `PullRequest.timelineItems`. We'll need to decide whether to surface all of them or a subset. We'll also need to check when this property became available and bump the minimum GHE version accordingly. (If it's not available in the GHE 2.x line then we'll need a graceful fallback.)
- We should coalesce events, perhaps by author (or also by type?). However, we need to make sure not to coalesce across other comments in the thread. For events that reference commit SHAs we should include a reference to the corresponding revision also/instead.
- We can convert the events into a new type of comment in the top thread. This type should be ignored when computing the discussion timestamp, and perhaps get a custom display component. GitHub has a rich display for events with links and hovers -- we could try to match that but it would be significantly harder than static, non-interactive synthetic comments.