Provide a way to mark pages as 'tested'

[WilcoFiers]

Feature suggested in EO (and myself previously):

There is currently no clear way you can indicate when you've finished testing a page. This can be problematic, especially with larger samples or if you do the audit over multiple days or share it with multiple auditors.

[nitedog]

Why is this closed? It seems still pending (for future versions).

[WilcoFiers]

Added a proposed example in #263, though this needs reviewing. I'm not happy with the way this looks yet.