Make HAML scrollable tabs consistent with GitLab UI scrollable tabs

Related to #345699

Currently scrolling tabs are implemented in the GitLab UI GlTabs component and in HAML using app/assets/javascripts/layout_nav.js#L38. However, these two implementations function differently:

GitLab UI GlTabs component

• Left and right arrows that are clickable.
• Can't scroll left and right, must click the arrows.

HAML version

• Left and right arrows that are not clickable. They are only used to indicate that there are more tabs.
• Must scroll left and right to see hidden tabs.