Gobierto Open Data: People exports

[amaia]

Connects to #303

What does this PR do?

Adds an open data page at /datos with the links to download data in csv and json formats for the GobiertoPeople module, and also an RSS feed for blog posts.
The page is linked form the people sidebar.

How should this be manually tested?

Check that the page exists at /datos and that you can download csv and json data and see the RSS feed

[codecov-io]

Codecov Report

Merging #528 into stable will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           stable     #528     +/-   ##
=========================================
+ Coverage   91.06%   91.16%   +0.1%     
=========================================
  Files         213      220      +7     
  Lines        5000     5050     +50     
=========================================
+ Hits         4553     4604     +51     
+ Misses        447      446      -1

Impacted Files	Coverage Δ
...rollers/gobierto_exports/application_controller.rb	100% <100%> (ø)
...ers/gobierto_people/person_statement_serializer.rb	100% <100%> (ø)
...ollers/gobierto_people/person_events_controller.rb	82.35% <100%> (+5.42%)	⬆️
app/models/gobierto_people/person_statement.rb	100% <100%> (ø)	⬆️
...p/controllers/gobierto_people/people_controller.rb	100% <100%> (ø)	⬆️
...obierto_people/person_event_attendee_serializer.rb	100% <100%> (ø)
...gobierto_people/content_block_record_serializer.rb	100% <100%> (ø)
app/models/gobierto_people/person_event.rb	100% <100%> (ø)	⬆️
...rollers/gobierto_people/person_posts_controller.rb	100% <100%> (ø)	⬆️
...rs/gobierto_people/person_statements_controller.rb	100% <100%> (ø)	⬆️
... and 20 more

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[furilo]

• Section title: "Descarga datos"
• URL slug: /datos
• Add the right link to the user sidebar in People module:

[ferblape]

I have completed a bit the test of the exports page. The rest of the PR is ok.