Branch protection settings should be aligned across repositories

[schemar]

All repositories use Travis CI for automated testing.
The branch protection settings of a repository govern, among other things, the requirements a PR has to meet before it can be merged.

These settings should be unified across all of our repositories.

The settings that must be decided on are:

• Should the same rules apply to all branches? (probably not; e.g. master more strict than develop)
• For different branches:
  • Dismiss stale pull request approvals when new commits are pushed?
  • Require review from Code Owners?
  • Restrict who can dismiss pull request reviews?
  • Require branches to be up to date before merging?
  • Require signed commits?
  • Include administrators?
  • Restrict who can push to matching branches?

These question need to be answered and the answers need to be documented in a file in the developer guidelines repository (does not have to be part of the main README, as it is very specific).