Consistent usage of moment add and subtract

In later versions they’ve deprecated the signature (period, number) so
following that guidance.
Upgrade moment dependencies

These showed up as sufficiently slow when profiling that I was
concerned. This is probably the headline change.

https://github.com/moment/moment-timezone/commit/803d2e2e936acf79b709ee5b5bb584cccd58e460
Merge pull request #850 from alphagov/performance

Various nudges to improve HTML response times