Add custom buttons to the header

**Is your feature request related to a problem? Please describe.**

I have a client with a workflow in which they are making changes constantly. In an effort to reduce the number of builds, we'd like to manually control when the site gets deployed. It makes sense that this trigger exist right alongside the content management experience.

**Describe the solution you'd like**

This is _similar_ to #4424. I'd like to be able to add a button to the header, ideally globally. It would be great if it could be a React component so I could also control the click event handler.

**Describe alternatives you've considered**

I'm considering two other approaches:

1. A browser extension.
2. A custom field that enables publishing only when _within_ some editable item.

I don't love either of these, as it's one more thing to manage, when it seems like this could be a valuable addition to the CMS application. However, I'm going to pursue both as an interim solution.

Start by reviewing the header and content-management experience, then read the related issue #4424 for existing context. Done would mean a globally configurable header button can be supplied as a React component and invoke a custom click handler, including the requested deployment workflow.

Written by the indexing model from the issue text.