Add honeypot to Source Interface to stop very basic spambots

[legoktm]

Status

Ready for review

Description of Changes

Add a hidden, empty input field to the Source Interface form that will
abort if anything is submitted in it. This will hopefully stop the most
basic of spambots that fill in spam in every form field they see and
submit the form. It is unlikely to stop any spambot specifically
designed for SecureDrop, as those will adapt as necessary.

This is inspired by and copied from MediaWiki's "SimpleAntiSpam"
functionality originally written by Ryan Schmidt[1].

[1] https://www.mediawiki.org/w/index.php?oldid=797352

Fixes #6295.

Testing

• Create a new source and submit a dummy message in the SI, observe that it works fine
• Use the browser's inspector to disable the "display: none", write something in the antispam input field, and submit. You should get a 403 error screen.

Deployment

No concerns.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container
• I have written a test plan and validated it for this PR
• These changes do not require documentation

[legoktm]

The HTML lint check is failing with:

securedrop/source_templates/lookup.html:61:10: Error: Separation of concerns: CSS should be defined in its own file: Move the contents of the "style" attribute to its own CSS file.

Putting the CSS inline is intentional, it ensures the field is always hidden, regardless of other CSS rules or if CSS fails to load for whatever reason. That said, I don't think it's a big loss to put it in the CSS and can do so if others think we should.

[zenmonkeykstop]

👍 to managing it via SASS like the rest of the CSS. Don't want to end up with a bunch of unnecessary linting exemptions

[legoktm]

Latest version renamed the form field to a boring "text", while keeping the name "antispam" server-side. And moved the inline styling to CSS.

[zenmonkeykstop]

Test plan looks good, code changes are minimal, field name is sufficiently boring. 👍