The diff page on the ouwiki is not very usable with screenreaders. We are going to make the following changes:

1) Show a different information message at the start when there are no changes, or when there are no changes to words (ie only html changes that don't get marked by the diff system).

2) Standardise 'added' and 'deleted' display sso these both use images before and after. Add a sentence (visible only to screenreader users) that tells people this is happening. This is supposed to let screenreader users know that they can use their 'skip to image' shortcut to jump to the next change.

[Note: I changed an existing language string, advice_diff, to add this hidden text. Sorry.]