Improve UX of YUM/DNF download form

The form on https://www.postgresql.org/download/linux/redhat/ allows users
to select their platform, architecture and database server versions, and
then generates a number of commands for them to run.

At present, each command is shown individually, requiring the user to
cut/paste multiple times.

The attached patch changes that to create a single commented script which
can be copied, pasted and run in one go speeding up and simplifying the
process for the user. The attached screenshot shows what it looks like.

Further improvements I'm considering are a Copy button and maybe some
colorization.

If the general principle is agreed, I can make similar changes for the
Debian/Ubuntu form.

--
Dave Page
Blog: http://pgsnake.blogspot.com
Twitter: @pgsnake

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