Add private fields to proposal components

[froger]

🎩 What? Why? This PR gives admins the ability to define as many private fields as are considered necessary, and also delete the fields defined as private in the proposals component at the end of the process, without deleting the rest of the content proposal or the authorship.

📌 Related Issues Same as #307, but using a branch where decidim-awesome contributors can make updates. Related to #? Fixes # Add private fields to the Proposals component

Testing

📷 Screenshots proposal private fields menu Disclosure for private fields

♥️ Thank you!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 53.93586% with 158 lines in your changes missing coverage. Please review.

Project coverage is 67.87%. Comparing base (a851688) to head (457bd45). Report is 60 commits behind head on develop.

Files	Patch %	Lines
...m_awesome/proposals/proposal_serializer_methods.rb	28.94%	27 Missing :warning:
...m_awesome/proposals/application_helper_override.rb	8.69%	21 Missing :warning:
...decidim_awesome/add_proposal_type_custom_fields.rb	43.33%	17 Missing :warning:
...cidim/decidim_awesome/has_proposal_extra_fields.rb	36.00%	16 Missing :warning:
...forms/decidim/decidim_awesome/admin/config_form.rb	17.64%	14 Missing :warning:
...idim/decidim_awesome/amendments_helper_override.rb	8.33%	11 Missing :warning:
...awesome/api/types/translated_custom_fields_type.rb	64.00%	9 Missing :warning:
..._awesome/proposals/proposal_serializer_override.rb	57.89%	8 Missing :warning:
lib/decidim/decidim_awesome/engine.rb	76.47%	8 Missing :warning:
...wesome/proposals/admin/update_proposal_override.rb	78.57%	3 Missing :warning:
... and 13 more

:exclamation: There is a different number of reports uploaded between BASE (a851688) and HEAD (457bd45). Click for more details.

HEAD has 6 uploads less than BASE

| Flag | BASE (a851688) | HEAD (457bd45) | |------|------|------| ||8|2|

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #314 +/- ## ============================================ - Coverage 96.02% 67.87% -28.15% ============================================ Files 97 75 -22 Lines 2440 2014 -426 ============================================ - Hits 2343 1367 -976 - Misses 97 647 +550 ```

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