remove SUMO tarballs, rewrite "desert island" build instructions

[warner]

Now that we're on pip, the Right Way to do a "desert island" build is to do a regular build before you get shipwrecked, then use pip's http/wheel cache on the island. We don't need separate tarballs for this.

I'd like to walk through our dependency tree and check that everything is available on pypi (as well as surveying which platforms have binary wheels available), but then I think we can stop hosting copies of our dependencies for the new release.

The actionable steps are:

• change the buildbot to stop generating SUMO tarballs
• stop advertising the tahoe-deps tarball
• change setup.cfg to remove the find_links= clause that points to misc/dependencies, tahoe-deps, and ../tahoe-deps (leaving the ones that point at tahoe-lafs.org for now, see #2750)
• add docs/desert-island.rst with a description of pip's cache, its default behavior, pointers to the pip docs to control its behavior, and instructions on what to do before and after the shipwreck (or getting on an airplane, or other offline development environment)

[Brian Warner <warner@…>]

In 1fb7cc7/trunk:

Makefile: stop producing SUMO tarballs

refs ticket:2752

[Brian Warner <warner@…>]

In 4f98c1d/trunk:

setup.cfg: stop using tahoe-deps/

refs ticket:2752

[Brian Warner <warner@…>]

In 17886c5/trunk:

add docs/desert-island.rst

refs ticket:2752

[warner]

I think that's all of it.