Docs style fixes: ``FOO`` vs `FOO`

I chose the option that is used more often in documentation:

         `True`, `False`, `None` for constants
         `GET`, `POST`, `HEAD` for HTTP methods
         `app.py`, `pip`, `requirements.txt` for filenames

But:

        ``DEBUG``, ``TESTING`` for config options
        ``X-Forwarded-Host``, ``X-Forwarded-For`` for HTTP headers
        ``template.html``, ``layout.html`` for Jinja templates

And:

        HTTP, not `HTTP`

During edits sense was not injured.