Consolidate mobile+desktop tab styling

When [bug 41661](https://bugz.meetup.com/show_bug.cgi?id=41661) launches, mobile and desktop tabs will share markup structure, which means we can consolidate their styles (which used to be a combination of Sassquatch tabs and crusty mobile/_global.scss styles).

*Note: this update removed left-padding from mobile tabs which is desirable for future tab implmentations, but will require some `leading-left` markup for existing mobile tabs. Must coordinate this PR with a cleanup bug release.