Enable require pull request reviews before merging in GitHub

[lukemckinstry]

Feature

Under consideration is to enable the feature which is built in to GitHub to require an approval on a PR before it can be merged (see feature docs here). When enabled the approval status is shown in the "Checks" section at the bottom of a PR.

We discussed that the current team practice is to review and approval all PRs before merging, as described in the Cesium contributor guide, so this enabling this feature would just effectively formalize an existing process.

Some feature details to highlight:

• When the reviewer requests changes, the same reviewer must then later approve or withdraw the request. Alternatively an overriding reviewer can nullify the request and approve the PR.
• There is a mechanism in place to automatically dismiss stale pull request approvals. I don't think we should enable this.
• As is the case currently, reviewers can self-assign themselves to a PR
• If we ever use adopt code owners, we can require reviews from code owners

[ggetz]

I've enabled this rule in the admin settings, and have allowed overrides for maintainers.

Please be on the lookout for how this is working out, and let us know here if there is any feedback.