Disables submit buttons on billing / upgrade page for demo
organizations since they will need to become permanent
organizations before upgrading to Zulip Cloud Standard.
Also creates an alert banner on the same page that links to
the help center article on demo organizations.
Updates sub-headers on demo organizations help center
article to match link text and to follow general convention
of using imperative verb forms in help center subheaders.
Part of #19523.
Co-authored by: Lauryn Menard <lauryn@zulip.com>
Tim Abbott
Eeshan Garg
Anders Kaseorg