Adding the ability to close GitHub Discussions

GitHub Discussions now supports the ability to close a Discussion. Discussions can be closed for one of three reasons: Resolved, Outdated, or Duplicate. Closing a Discussion is much like closing an Issue or a Pull Request. Users can select a reason for closing in the dropdown. A screenshot of the dropdown is shown below:

The reason for closing is visible to users in two places on the page.

First, in the icon at the top of the Discussion:

Second, on the events timeline:

Besides the state of the Discussion being visible on the page, we're also now surfacing the state of a Discussion in search. We're adding three new filters:

is:<closed/opened>
filters out open/closed discussions

reason:<resolved/outdated/duplicate>
returns closed discussions that were closed with the provided reason

closed:<date>
returns discussions closed on a certain date. Supports < and > operators to get discussions closed before or after the date.