Add uninstall command; fix other phonies

`make uninstall` can now be used to remove (only) installed files. Note
that it only removes files which correspond to presently‐extant source
files; if a source is removed, then `make uninstall` will not remove
any previously‐installed derivatives.

`make all` used to compile assets, but this is unnecessary.

`make clean` now guards against `BUILDDIR` being empty (which would
otherwise result in a dangerous `rm -rf /`).