Implement consultation response reports endpoint

[ferblape]

Connects to #359

What does this PR do?

This PR adds a new endpoint with the export of the responses of a consultation in CSV.

[codecov-io]

Codecov Report

Merging #532 into stable will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           stable    #532      +/-   ##
=========================================
+ Coverage   90.98%   91.1%   +0.12%     
=========================================
  Files         211     213       +2     
  Lines        4978    4993      +15     
=========================================
+ Hits         4529    4549      +20     
+ Misses        449     444       -5

Impacted Files	Coverage Δ
app/forms/gobierto_admin/census_import_form.rb	100% <ø> (ø)	✅
...s/consultations/consultation_reports_controller.rb	100% <100%> (ø)
...ices/gobierto_budget_consultations/csv_exporter.rb	100% <100%> (ø)
app/models/gobierto_budgets/budget_line_stats.rb	85.91% <0%> (+1.4%)	✅
app/models/gobierto_budgets/site_stats.rb	79.16% <0%> (+2.77%)	✅
app/controllers/gobierto_admin/base_controller.rb	100% <0%> (+6.45%)	✅

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