Dynamic stylesheet includes with Webpacker

[ahukkanen]

🎩 What? Why?

The Decidim asset compilation will fail if all Decidim gems are not available in the bundle because the application.scss assumes all these gems are available:

decidim/decidim-core/app/packs/stylesheets/decidim/application.scss

Lines 4 to 12 in 6bd8053

	@import "stylesheets/decidim/accountability/accountability";
	@import "stylesheets/decidim/budgets/budgets";
	@import "stylesheets/decidim/conferences/conferences";
	@import "stylesheets/decidim/consultations/consultations";
	@import "stylesheets/decidim/initiatives/initiatives";
	@import "stylesheets/decidim/elections/elections";
	@import "stylesheets/decidim/votings/votings";
	@import "stylesheets/decidim/proposals/proposals";
	@import "stylesheets/decidim/surveys/surveys";

This works fine when Decidim is installed from GitHub (because they are all available in the repository) but it fails when it is installed e.g. through RubyGems in the future.

See further explanations from #8109.

This PR proposes a solution to this problem by moving the stylesheet import configurations to the module's assets.rb.

📌 Related Issues

• Fixes Webpacker compilation fails if all core gems are not available #8109

Testing

See #8109

📋 Checklist

• ❓ CONSIDER adding a unit test if your PR resolves an issue.
• ✔️ DO check open PR's to avoid duplicates.
• ✔️ DO keep pull requests small so they can be easily reviewed.
• ✔️ DO build locally before pushing.
• ✔️ DO make sure tests pass.
• ✔️ DO make sure any new changes are documented in docs/.
• ✔️ DO add and modify seeds if necessary.
• ✔️ DO add CHANGELOG upgrade notes if required.
• ✔️ DO add to GraphQL API if there are new public fields.
• ✔️ DO add link to MetaDecidim if it's a new feature.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.