moved default settings out of instance configuration

previously each instance's configuration included the full list of
twanager and system plugins as well as Markdown extension settings -
which complicates upgrading BFW

now all that's required there is BFW (as both twanager and system
plugin), plus the secret of course

unfortunately though, this meant abandoning the declarative structure
for BFW's plugin dependencies (tagdex and static) and manually
initializing them in BFW's `init` instead (where distinguishing between
twanager and system plugins is far less elegant)