Replace born_at with token in Voters' census data

[andreslucena]

This PR brings 3 changes:

1. We change the name and type of one of the Census fields: as date of birth (born_at) isn't a data that we have for the pilot that we'll do in 10 days we'll use "tokens" (strings of texts). By design we don't validate them (as they could be any string with any length)
2. We allow the vote overwrite and show a callout message when that's the case

Testing

Census with token

1. Create a new census file with the header "email;token"
2. The token can be any string
3. Do the whole process (specially setup and voting, as they're the tasks where the census is used)

Vote overwrite

1. Vote once
2. Vote again
3. See callout

[codecov[bot]]

Codecov Report

Merging #48 (df709ed) into main (113fba2) will decrease coverage by 0.02%. The diff coverage is 100.00%.

:exclamation: Current head df709ed differs from pull request most recent head 3f8361b. Consider uploading reports for the commit 3f8361b to get more accurate results

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   86.80%   86.78%   -0.02%     
==========================================
  Files          80       80              
  Lines        1508     1506       -2     
==========================================
- Hits         1309     1307       -2     
  Misses        199      199              

Impacted Files	Coverage Δ
...ms/decidim/vocdoni/admin/census_credential_form.rb	100.00% <100.00%> (ø)
app/forms/decidim/vocdoni/login_form.rb	100.00% <100.00%> (ø)
app/models/decidim/vocdoni/answer.rb	100.00% <100.00%> (ø)
app/models/decidim/vocdoni/csv_census/data.rb	95.65% <100.00%> (ø)
app/models/decidim/vocdoni/voter.rb	92.85% <100.00%> (ø)

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