Set HOME to @sitedir@/data by default.

Most web servers don't have a HOME variable by default, or it points somewhere
unwritable. We end up needing HOME for the .ssh directory, though.

This change sets HOME to the new @sitedir@/data directory by default. This
must be writable by the web server, so instructions at the end have been
updated (and made a little clearer).

This new directory is also the new location of the SQLite DB file for new
installations. This won't affect existing installations in any way.

Existing users will have to manually update their Apache configuration, if
using mod_python, and update the permissions on the directory.

Created a new site and saw the new directory, text, and Apache configuration.

Upgraded an existing site and saw the upgrade notification. Upgraded it again and didn't see the notification a second time (since the data directory already existed).