Unify user permission texts

We had the same texts four times, with slight variations in the case of
the management section.

We're unifying them under the "verification" i18n namespace, since the
texts are about actions which can be done depending on whether users are
verified or not.

Note the names of the i18n keys aren't very consistent, since we use
"debates" in plural but "proposal" in singular. We're leaving it like
this so existing translations aren't affected.

spec/system/management/managed_users_spec.rb

@@ -34,7 +34,7 @@ describe "Managed User" do
       fill_in "document_verification_document_number", with: user.document_number
       click_button "Check document"
 
-      expect(page).to have_content "Vote proposals"
+      expect(page).to have_content "Participate in final voting"
 
       click_button "Verify"