Move style from index.php to RiDi.css
Makefile: install -> install_nginx_debian
Makefile: add default target that prints existing targets
README.md: fmt(1) the paragrah
index.php: show proposed groups: cosmetic changes

Mainly, wrap long lines and get rid of spaces before
functions invocation.
index.php: count number of subscribers per group

Count the number of users into a group, and add the number on the right of
each group. Allows you to know the number of users per group at a glance
(i.e. you don't need to inspect each group anymore).

Trust that $row[0] is an integer, since we know it's an integer from
the database schema.

While there, make it more explicit that we sanitize the title that was
stored into the database.