quickstart.rst: use out-of-line links

[zooko]

When I run rst2html --verbose quickstart.rst > quickstart.html, it gives me complaints about the in-line links, like this:

$  rst2html --verbose quickstart.rst > quickstart.html
quickstart.rst:3: (INFO/1) Duplicate explicit target name: "the tahoe-dev mailing list".
quickstart.rst:5: (INFO/1) Hyperlink target "the tahoe-lafs project" is not referenced.
quickstart.rst:5: (INFO/1) Hyperlink target "about tahoe-lafs" is not referenced.
quickstart.rst:19: (INFO/1) Hyperlink target "the tahoe-dev mailing list" is not referenced.
quickstart.rst:29: (INFO/1) Hyperlink target "the python download page" is not referenced.
quickstart.rst:43: (INFO/1) Hyperlink target "download the latest stable release, v1.9.1" is not referenced.
quickstart.rst:69: (INFO/1) Hyperlink target "running.rst" is not referenced.
quickstart.rst:77: (INFO/1) Hyperlink target "advancedinstall" is not referenced.

With the attached patches, all those complaints go away. I also edited the text a little bit.

This patch includes all the patches from #1703, #1702, #1701.

[davidsarah]

• I agree with making the links to external URLs out-of-line, but it seems unnecessary and more verbose for the links to about.rst and running.rst. To eliminate the warning from rst2html --verbose, use an ​anonymous hyperlink reference ending in two underscores, like this: `<running.rst>`__
• There's a long line on line 24 of the diff.

[zooko@…]

In 638dcf867aaf1eb2:

docs: quickstart: edits

• use out-of-line links to avoid a warning from rst2html --verbose (fixes #1704)
• reflow to 77 fill-column and prepend utf-8 BOM (fixes #1703)
• recommend Python 2.7 (fixes #1702)
• remove link to wiki:AdvancedInstall (fixes #1701)