Table Block: Scrolling is not evident when a browser/device hides scrollbars.

[kjellr]

Describe the bug

On browsers/devices that hide scrollbars, there's no visual indication that the table block can scroll horizontally. We might consider adding some sort of inner drop shadow to make that more clear.

Screenshots
If applicable, add screenshots to help explain your problem.

Safari on iOS 11.4.1:

Safari 11.1.2 on Mac OS 10.13.5:

Suggestion

Additional context

Related: #8325, #7963

[kjellr]

Worth noting: Since this comes up more than once (#7963), we may want to establish a design pattern to account for scrolling content like this.

[kriskarkoski]

Noticed during recent Gutenberg testing on a Mac with scrollbar display disabled

[afercia]

See proposed scroll indication on #16324 (comment)