These events have been superseded by Review and its State field.

This check is now performed upstream in maintner,
so no need to do it here.

Follows https://golang.org/cl/107296.
Updates https://golang.org/issue/21984, https://golang.org/issue/22060, https://golang.org/issue/24875.

Similar to https://github.com/shurcooL/issues/commit/697ee8e877735feed6151525cad859327bfe40c4.

Follows https://dmitri.shuralyov.com/state/...$commit/28bcc343414c6adcd7b6911f9d0ef1ad6fbf30ae
and https://github.com/shurcooL/events/commit/25a0212309664cda69930a58f8369f24b9029812.

Change timeline item ID type from uint64 to string.

Rearrange services.

There's no longer a clean distinction between comments and events.
There are other timeline item types that don't fit into either category
cleanly. Also, underlying implementations for various APIs are very
similar and have overlap. As a result, it makes sense to merge these
two methods into one.

Remove ListCommitsOptions from ListCommits. Expected usage is to always
get all commits, then find the one you're interested in. This is what's
needed to implement prev/next commit buttons, and more closely matches
what's returned by 3rd party APIs.

The button styling can be improved; to be done later.

Add GetDiff method to it.