Allow exploring context from pages

[Niols]

eg. go to previous/next version when viewing the version from a search

[Niols]

Almost in a good enough state to merge and get the functionality, but there are some follow-ups that are worth pursuing:

• (feature) recognise left/right arrow to go to previous/next element
• (feature) recognise swipe left/right to go to previous/next element
• (performance) cache search on server side to avoid searching again
• (performance) add an endpoint to get just one element and its neighbours
• (quality) add tests

[Niols]

Added two tests and opened issues for all the other points; everything is now ready to merge.