Display boards filter bar on mobile

What does this MR do?

Fixes #45864 (closed)

On the issues -> boards page, displays the filter bar on sm and xs screen sizes.

MR for EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14663

Be warned, ye who enter here! This MR does not contain the best CSS and HAML ever written. It contains the minimum viable change for a component that is due to be refactored, and as a result some of the CSS and HAML look like spaghetti. Bring your own sauce.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers