Trigger manually sync/full sync

## Prerequisites:

* Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.

* Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).

* If the functionality is not yet available in the API, it would be helpful if you
contacted support (https://support.github.com/) or posted in the Community Forum (https://github.community/). Please
include a link to the forum post if you create one or a copy of the response from support.

## New Feature

Imagine you have the situation where the safe-settings app did not configure a repository properly, because for whatever reason, e.g., webhook not send to the app, an application error, etc..

Wouldn't it be great if there would be at least the possibility to trigger manually a sync or full sync through a dedicated endpoint?