Allow access to about page even with no index patterns

The about page doesn't rely on index patterns being set up, and administrators have requested that we allow access to this page when no index patterns are set up, to see the version number.

In the future, if we want more pages to be accessible with no index patterns, we might want to make an array of `allowedRoutes` and check if the current route matches any of them. But this doesn't seem too likely, so I just added another check to see if the current route matches `settings/about`.

Closes #2073.