Better usage text for rename-app

Make it very clear that we want a "Name", not a "name".
Upgrade to the latest jQuery Form plugin
"CREATE EXTENSION" statements must be run as the superuser

We take the very slight risk of assuming that 000-extensions.sql will
always be the first file in schema/*.sql and dealing with it specially.
Merge INSTALL and requirements
A few small tweaks, and a reference to schema/README
Use ->server->url instead of deprecated ->app_url
Use ->attr('value') instead of deprecated attrs->{value}'
Don't complain if the previous test left the table lying around
Expect text/plain;charset=UTF-8 instead of text/plain
Clean up once and for all as dbadmin rather than piecemeal
Don't try to drop sprockets as dbadmin, though
Make sure the token exists before trying to access it
Extend form_for and make post_form a trivial wrapper around it
Use form_for when generating forms for named POST-only routes