Harden Github Settings for Branches/Repo Access

[nce]

We need to harden a few settings in Github. This puts a few restrictions on the contributors, but we figure this is necessary.

AC:

• [x] protect main branch from individual/nonreviewd commits
• [x] force signed commits on all contributors on main
• [x] Require 1 approver (not himself) on every PR

[nce]

Implemented.