Hide dashboard edit mode on small screen sizes

[timroes]

Summary

With #29896 we'll introduce a responsive mode for dashboards, where on screen sizes s and xs all panels will just be shown full with below each other (so making the grid a list). Since we cannot really edit that dashboard anymore in that configuration, since it relies on the grid to be present, we decided to remove the Edit button on small screens.

This PR adds the capability to set a custom CSS class to kbnTopNav items and uses it to hide the edit button on s and xs screens.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• [ ] This was checked for cross-browser compatibility, including a check against IE11
• [ ] Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• [ ] Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• [ ] This was checked for keyboard-only and screenreader accessibility

For maintainers

• [ ] This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-app

[cchaos]

Works for me! TY!

[elasticmachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request

[flash1293]

Code LGTM!