GlTabs: Add horizontal scrolling interaction

Based on our Figma design guidelines, we have a "scrolling" interaction with tabs if they go wider than their parent container. This issue serves as a SSOT for adding this behavior in gitlab-ui.

Screen_Shot_2020-12-14_at_12.33.29_PM

Open for MRs.

cc @tauriedavis

Edited by Jarek Ostrowski