Link to previous elections on ballot page

[pmk01]

Jon F says: Results from previous elections" section to each election page on Who? Just some links to previous years would be great - if I was undecided on who to vote for then I'd want to see previous results to know which parties came first and second, and how close it was.

Part of this (or an initial piece of work) could be turning the results table on each candidate's profile into links to the relevant ballots - see also #942

We might also want to make it clear on SV & STV ballots that we don't collect results for these.