Feature: filter branches based on a prefix when using GitHub storage mode

[JPrisk]

Issue: working on a project where there are lots of branches (and there are expected to be many more) - currently it is hard for content people to differentiate between their branches and general development stuff.

Suggestion: provide a GitHubConfig option as handler to filter branches

Examples:
feature/content... if wanting to isolate development/content PRs
feature/[project-prefix] when working in a monorepo

[JedWatson]

This seems like a great idea. We should take it into account when creating branches through the Keystatic UI as well, so that when you create a new branch from within Keystatic, it shows up.