Use react-router for legal page tabs.

[lindapaiste]

Fixes #2621

The primary purpose is to control the routing of the Legal page declaratively (with routes) rather than imperatively (history.push). The react-tabs package is fine on its own but doesn't work elegantly alongside react-router.

Changes:

• RouterTab
  • Extracted the Tab component from DashboardTabSwitcher.jsx into a reusable RouterTab component.
  • Remove the isSelected prop and use the react-router NavLink component which automatically detects if a link is for the current page.
• _dashboard-header.scss
  • The NavLink means that the dashboard-header__tab--selected class is now applied to the child <a> rather than the parent <li>, so I had to move around some styles in the .scss file to keep things correct.
• Legal
  • Use the RouterTab instead of the react-tabs package.
  • Made the Legal component into a container that renders the correct content based on props, rather than switching children.
  • Move the policy fetch useEffect and the Helmet up into the parent since it is the same for all tabs.
  • Add a Loader while loading. // TODO: this is not visible and should probably be a child of the PolicyContainer.
  • PrivacyPolicy etc. are now specific instances of Legal.
• routes.jsx
  • Render the specific page for each URL. This eliminates the need to look at the path inside the Legal component.
• DashboardTabSwitcher
  • This should look identical but there are some minor changes to the HTML structure:
    • I swapped the div and ul tags so that the li elements are direct children of the ul.
    • There is no more h4 in each tab. Just <li><a/></li>. I don't think this matters? But now that I'm writing this I think I might put it back?
    • The active tab uses an a instead of a span.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[PiyushChandra17]

Looks good 👍

[raclim]

This is great, thanks so much for working on this!