allow_force_push flag is always disabled, with no way to configure

## Problem Description
When branch protection is applied/enforced by safe-settings, the allow_force_push flag is always reset to 'disabled', and it appears there's no way to configure this setting for a repo.

### What is actually happening
See above

### What is the expected behavior
Wither 'allow_force_push' should not be changed if it is not configured by safe settings, or there should be a way to configure it in the repo protections file.

## Context

### Are you using the hosted instance of probot/settings or running your own?
Running our own

### If running your own instance, are you using it with github.com or GitHub Enterprise?
github.com

#### Version of probot/settings
1.1.9

#### Version of GitHub Enterprise
N/A