Add the display of Total Contributors "for all time" to the main stats page

[ebarry]

Reposting a cleaned up issue for @kryptc ! Taken from #3152 and part of broader project at #3498

The https://publiclab.org/stats page shows a lot of wonderful information such as

5170 research notes have been posted by 1261 contributors since almost 8 years ago

I would like to see the total number of unique contributors for all time. Exactly as is shown https://publiclab.org/stats/range/1-1-2011/25-07-2018

But as the default view at https://publiclab.org/stats

IMPORTANT: it's a big query, so we'll need to do a weekly cache of this data so it's only generated weekly. See how caching works here:

plots2/app/controllers/home_controller.rb

Line 163 in 062ff10

Rails.cache.fetch("front-activity", expires_in: 30.minutes) do

Please show us where to look

The stats page takes logic from:

https://github.com/publiclab/plots2/blob/master/app/controllers/stats_controller.rb

Please feel free to ask for any doubt or help.

Thank you