ui: add support for a tweakdefaults knob
We've been talking for years about a one-stop config knob to opt in to
better behavior. There have been a lot of ideas thrown around, but
they all seem to be too complicated to get anyone to actually do the
work.. As such, this patch is the stupidest thing that can possibly
work in the name of getting a good feature to users.
Right now it's just three config settings that I think are generally
uncontroversial, but I expect to add more soon. That will likely
include adding new config knobs for the express purpose of adding them
to tweakdefaults.