auto-update a PR from `master` when no conflcits

[danisharora099]

Problem

When PRs are opened, they need to be manually rebased/merged with master if they fall behind.

Proposed Solutions

Similar to how Dependabot handles its own PRs by rebasing with master, branches with open PRs should auto-rebase/merge.

[weboko]

This is a nice to have feature as current mitigation is:

• press Update button in a PR;
• pull master to a PR and sign commit;

I suspect it might not be possible to do this enhancement as it is would require creating some bot that will monitor PRs and will do a rebase against master. AFAIK there is no configuration for it in Github

Marking as good first issue as it doesn't require much knowledge of protocols.