define branch protection rules for modernapps.github.io repo

[afewell]

Branch protection rules should be put in place that are minimal given the current light staffing, but also put some checks and balances in place to help limit unintended issues.

Proposal:

• Force pushes should only be allowed by Maintainers/Administrators and should not be used outside of extenuating circumstances.
• Pull requests from a nonprotected branch should be used for all updates
• At this time pull requests should only require 1 review
  • Any member with collaborator or higher privilege should be able to approve their own review

[afewell]

I have implemented the branch protection ruleset per the proposal