Reposition Test Plan Actions

[jkva]

This PR addresses the following item from PAC's "ARIA-AT App Screen Reader Accessibility Observations" document:

For non-admin users (i.e. testers), the "Actions" column is empty when a test plan isn't assigned to anyone. This reduces the discoverability of the " Assign Yourself" button, which is arguably a test plan action but is confusingly located in the "Testers" column. When a tester is assigned, the " Start testing" link is located in the "Actions" column as expected, but these inconsistencies make the table quite unpredictable to use.

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Effective changes:

• Move the "Assign Yourself" button into the "Actions" column;
• move the "Assign User" pop-up widget into the "Actions column;
• rename "Remove" button to "Remove Plan"; and
• adjust color contrast for "danger" style.