Used fixed widths and ellipsis for variable table styles

The project variables UI was moved from its own page to the CI/CD settings page. Because there are some differences in the overall page layout, the space allotted for the variables table is smaller, so the responsive styling for this table wasn't being applied soon enough.

This is a tricky problem because we allow for keys and values of highly variable length, yet we try to style the table for both revealed and shown values. In this MR, I switch to using ellipsis to contain the length of the values and the keys, thus normalizing the layout across states.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25780 again

cc: @jschatz1