Make tab-nav a bit more responsive

[tomalec]

Changes proposed in this Pull Request:

This PR makes the tab navigation a bit more responsive, my making tabs wrap when the container is too narrow. It also adds the bottom "bar" to the items, sot when the elements wrap, the "bar" is visible under all tabs.

Closes #2473

Replace this with a good description of your changes & reasoning.

Screenshots:

Before

After

Detailed test instructions:

1. Go to G4W dashboard /wp-admin/admin.php?page=wc-admin&path=%2Fgoogle%2Fdashboard
2. Change the viewport size to something narrower
3. Check that all the tabs are visible
4. Go to Product feed /wp-admin/admin.php?page=wc-admin&issueType=product&path=%2Fgoogle%2Fproduct-feed
5. Check that all "Issues to resolve" tabs are visible.

Additional details:

1. We may consider implementing more dedicated UI for the narrower screens, but I believe it's a good enough quick fix in the meantime.

Changelog entry

Fix - Make the tab navigation tabs wrap when the screen narrows.

[eason9487]

Thanks for the fix. LGTM.

The only comment should not need another round of code reviews.