Improve mega menu submenu highlighting

[chosak]

We want a mega menu submenu to be highlighted (i.e. appear visually selected) when users are on a child page of that submenu.

The current logic assumes that we can determine this by comparing URL paths; for example, if a submenu contains a link to /foo/, and the user is on /foo/bar/, then we should highlight the submenu.

For increased flexibility, we also want to support the case when menu items point to links with query strings, for example /foo/?something. In this case, we want a page like /foo/bar/ to also mark the submenu as selected. This commit alters the logic so that we strip off the menu link query string before comparing it to the request path.

Note that this logic only applies to the top-level highlighting of submenus (for example Consumer Tools on the current site). This doesn't alter how the specific menu links themselves are highlighted.

How to test this PR

1. Run locally and confirm that menu highlighting works properly with the current menus.
   • For example, the Press Resources page should highlight both the About Us submenu as well as its parent Newsroom link.
2. Change a menu link so that it points to a URL with a query string (see Add management commands to export/import mega menu #6011 for one way to do this), and then confirm that its menu continues to be highlighted if you visit the URL without the query string.
   • Note that the link itself will not be highlighted; this is unfortunate but deliberate to avoid duplicate highlighting in cases where multiple links differ only by their query string.

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code follows the standards laid out in the CFPB development guidelines