Change/add text on contribution confirmation page

As a site administrator
I need to add/change text on the contribution confirmation page
so that end-users understand the context/purpose of the page

E.g. we are using contribution/memberships for magazine subscription management. From an end-user perspective, the text about 'contributions' may be confusing, although technically accurate. We would like to modify the text on that page, including the possibility of adding a header/footer text, to help our users understand the purpose of the page.

See also: CRM-15648