Link colours

This adds the feature about the colour of the links – red or blue – at the time of the Memento.

It has been quickly tested on one example (so more testing is probably needed), and more work is needed for the details (see @fixme: green and blue italics colours, and deleted-then-recreated links, and check the parser cache status about the link colours).

I will understand if you prefer not to merge now, to check the parser cache status and wait more testing. In any case, I will try to continue to work on this feature. As a side note, I plan to incoporate the work I did on my extension BackwardsTimeTravel into Memento https://www.mediawiki.org/wiki/Extension:BackwardsTimeTravel#Deprecation.