Be more explicit about needing an existing database with permissions.

Some users get stuck during installation because they don't realize
they need a database pre-created. They likely don't read the text
briefly explaining it. It doesn't help later that we're vague on the
permissions needed.

Now, we have a "NOTE: " prepended during text installation and a Warning
icon during GTK+ installation saying a database must already be created
and to read the database documentation. We also are more clear about needing
*table* creation rights later on, and that permissions must already be
set on the given database.

Hopefully this helps to explain point things out for those users who may miss
it or be confused.

Tested the text and graphical installs and saw the new text and prefixes.