Makefile: change name of `all` target to `production`
Makefile: change name of `preview` target to `local`
template/main.mpp: add google analytics tracking code

Will be included only if `staging` var is set in MPP.
add `staging` target to build system
make `local` the default target
add robots.txt file except when building production site
Makefile: add `staging` to list of PHONY targets

Should have been done in 3d017d04bc7197077af03d57ee33ef368b5fdd4f.
Merge branch 'analytics' into redesign