Cosign improvements - batch 1

[sergei-maertens]

Closes #4320 (partly) The frontend aspect is handled in open-formulieren/open-forms-sdk#729

Email/PDF contents will be done in a different PR.

Changes

• Language: ondertekener -> ondertekenaar
• Detect if a link is used for cosign or not and return this information to the frontend
• Support "email with link flow" where the code is included in the link, bypassing the "enter the code" step. If the link is not in the request email, users still need to enter the code.
• Made the confirmation page title configurable (template), added configuration options for cosign confirmation page title and cosign confirmation page body. When cosign is enabled, the title + body templates for cosign are rendered, otherwise the "regular" ones. The body template supports a template tag for the cosign button to be displayed directly in the SDK/confirmation page.
• "Toon NIET het nummer indien links worden gebruikt" -> due to the template nature this can be configured by municipalities themselves. Our default templates still include it because it's bad customer support if they have no references at all.

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [x] Checked copying a form
  • [x] Checked import/export of a form
  • [x] Config checks in the configuration overview admin page
  • [x] Problem detection in the admin email digest is handled

• Release management

  • [x] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [x] Commit messages refer to the relevant Github issue
  • [x] Commit messages explain the "why" of change, not the how

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.92473% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.56%. Comparing base (c50aa99) to head (7078014). Report is 47 commits behind head on master.

Files with missing lines	Patch %	Lines
src/openforms/submissions/views.py	91.66%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4808 +/- ## ========================================== + Coverage 96.55% 96.56% +0.01% ========================================== Files 748 749 +1 Lines 25423 25499 +76 Branches 3362 3375 +13 ========================================== + Hits 24548 24624 +76 Misses 610 610 Partials 265 265 ```

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[sergei-maertens]

Seems good to me. The only thing I don't get is the "Cosign now" button that is shown when the user completes the form. I saw that the task has this as a requirement but why do we have it there since the cosigning step has to do with another user?

That's for the cases where the cosigner is physically present the original submitter - this makes it possible to finish the full submission more quickly.