Adx 977 invalid filename allowed upon upload

[A-Souhei]

Description

Having a % in the filename causes a 400 bad request.
What I did: wrote an helper to encode the filename in the CKAN template
Added a short javascript code in the template to update the filename upon DomContentLoaded

Checklist

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• The Jira ticket for this issue has been updated to "Ready to Review" or equivalent.
• I have developed these changes in discussion with the appropriate project manager.
• My code follows the general Fjelltopp documentation (see Confluence).
• I have made corresponding changes to the Fjelltopp documentation (see Confluence).
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• New dependency changes have been committed.
• I have added automated tests that prove my fix is effective or that my feature works.
• New and existing tests pass locally with my changes.
• My changes generate no new warnings.
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[tomeksabala]

I think you can fully replace your url_encode implementation with the use of Python urllib quote function (still wrapped in form of template helper of course).

Also please add a regression test for the bug.

[tomeksabala]

Please fix the linting (flake8) issues in the code.