Highlight roadmap items marked 'needs contributions'

[aryanyk]

Fixes #447

Description

This PR adds support for highlighting roadmap items marked with the "needs contributions" label.

Changes included:

• Added a NEEDS_CONTRIBUTIONS_LABEL constant to MilestoneItem.
• Added a needs_contributions cached property to detect when an item has the needs contributions label.
• Added an optional contributions_url field to allow editors to override the default link.
• Updated the roadmap template to display a "Contribute" badge for items marked with needs contributions.
• The badge links to contributions_url when provided, otherwise falls back to the issue URL.

The implementation follows the existing pattern used for sponsorship labels and keeps the changes minimal.

AI usage

AI assistance was used to help review the implementation approach, validate the template logic, and draft parts of the pull request description.(Claude Opus 4.6)

Output:

[vossisboss]

@aryanyk It looks like your PR didn't pass our ruff lint check. Could you please resolve that? Also, thank you for disclosing your AI usage.

[aryanyk]

Hi @vossisboss

I have fixed the linting error. Can you please re-run the CI to check .
Thank You

[vossisboss]

Thank you @aryanyk! One other thing that would be super helpful would be for you to rename the migration file so that it is a bit more descriptive. The automatically generated files don't always tell us at a glance what fields are being added or removed.

This code all works well on my local setup with no obvious errors. We are going to definitely need @thibaudcolas to test this PR though because he is the one who manages our roadmap items and he'll make the final decision on whether to include the flag you created.

It will probably be a while before Thibaud updates the Roadmap again and has a good chance to test this contribution (a few months possibly). So please be patient @aryanyk and know that we definitely appreciate you taking the time to make this for us.

[aryanyk]

Rename Done