Group/project members table has horizontal scroll on mid-width screen sizes

Summary

Group/project members table has horizontal scroll on mid-width screen sizes. This is because we added the Created on and Last activity columns in #349884 (closed) and there isn't enough room in the table to display these new columns.

Steps to reproduce

Go to a group or project -> Group/Project information -> Members. Resize the screen

What is the current bug behavior?

Horizontal scroll on mid-width screen sizes

What is the expected correct behavior?

No horizontal scroll

Relevant logs and/or screenshots

Screenshot

Possible fixes

Look into a "table builder" that allows users to select what columns they want to see. There was discussion about this in #232664 (comment 463724636) and gitlab-org/gitlab-services/design.gitlab.com#1214 (closed)

Proposal

Rearrange the members table to allow the row to fit within the space