This gets us out of the business of maintaining our own icons. The
milsymbol library generates the SVG data needed to display anything
covered by APP-6.
This is briefly moving us over to my fork of pydcs while we wait for
https://github.com/pydcs/dcs/pull/206 to be merged. The adaptation is
invasive enough that I don't want it lingering for long.
A possible explanation for the infrequent CTDs we've been seeing since
adding fast forward is that QWebChannel doesn't keep a reference to the
python objects that it passes to js, so if the object is GC'd before the
front end is done with it, it crashes.
We don't really like QWebChannel anyway, so this begins replacing that
with FastAPI.
