Try to prevent node upgrade via homebrew

escape html before parsing commonmark

I think the most recent build broke deployments?

Add settings option to display avatars as squares

Try to reduce memory usage by reusing avatar pixmaps

Mentions are now loaded from the cache instead of
directly from the web request.  Mentions are also
properly saved to the cache now (instead of as empty
strings).  Still lots of tweaks left on this feature.

On first launch, before the user has configured any settings,
check the value of the QT_QPA_PLATFORMTHEME environment var.
If it is set, use the system theme as the default instead of the
light theme.  This fixes #72.

Cache user mentions when they are retrieved from the server.
This logic currently isn't being utilized by the UI.  Additionally,
the app should use a 'since' value to only get mentions newer
than those stored in the DB, to avoid excessive web requests.

This will be implemented in a future commit.