serve: Add server sent events for showing pipeline errors in the debug window. The server can now run an endpoint that streams pipeline status, including errors or "fixed" statuses. As a result, I had to investigate using event-loop based server alternatives, before I figured out the correct flag to set in Werkzeug. Support for Gunicorn is therefore now possible, although disabled by default. I will come in handy though when proper support for CMS-mode is enabled.

PRIORITY_FIRST = -1
PRIORITY_NORMAL = 0
PRIORITY_LAST = 1


class Formatter(object):
    FORMAT_NAMES = None
    OUTPUT_FORMAT = None

    def __init__(self):
        self.priority = PRIORITY_NORMAL

    def initialize(self, app):
        self.app = app

    def render(self, format_name, txt):
        raise NotImplementedError()