Add CMS version number to header

see screenshot, added 'Hippo CMS 7.7.1' to the top header.
We could also change the text 'Welcome to CMS 7' to reflect the right version number.

This makes it easier to:

• for the end-user to identify the version when submitting a bug
• for hippo employees to identify the version if someone submits a screenshot / video.

Note that this won't show detailed information like the CMS, HST and repo version. A link to the admin panel would do the trick, however this area is not accessible for authors and editors.