Design improvements for activity pages, person diffs, and header nav

[mhl]

e.g. the Recent Changes page is just a table, that doesn't work well on mobile:

• http://yournextmp.mysociety.org/recent-changes

And the "Top Users" page just looks a bit inelegant:

• http://yournextmp.mysociety.org/leaderboard

In addition, once #39 has been done, there'll be lots of scope for presenting that nicely.

And there are also candidates with photos – see #17

I've asked for some time from our designers for this.

[mhl]

In addition, having added these to the nav bar at the top might be making it a bit crowded?

[mhl]

@zarino's redesign is deployed now (and looks great, thanks!) so I'll close this and we can open new tickets for other design requests.