User stats for admins

[ferblape]

Connects to #527

What does this PR do?

This PR adds a small snippet with users stats for admins.

[codecov-io]

Codecov Report

Merging #534 into stable will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           stable     #534      +/-   ##
==========================================
+ Coverage   91.06%   91.07%   +0.01%     
==========================================
  Files         213      214       +1     
  Lines        5000     5021      +21     
==========================================
+ Hits         4553     4573      +20     
- Misses        447      448       +1

Impacted Files	Coverage Δ
app/models/user.rb	100% <100%> (ø)	✅
app/controllers/gobierto_admin/users_controller.rb	96.87% <100%> (+0.32%)	✅
...ters/gobierto_admin/users/users_stats_presenter.rb	100% <100%> (ø)
app/models/gobierto_budgets/site_stats.rb	77.77% <0%> (-1.39%)	❌

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