Repair menu highlighting, both for main menu and submenus

During the recent improvements made on the website, the menus and
submenu highlighting for selected menus fell into disarray.
Correction requires changes to the CSS and templates both, because
Django doesn't really have a convenient way to set variables in
templates to use to solve this.

There are still a few submenu items highlighting not working even
after this commit. More work needs to be done.

Filings ⇒ Transparency, necessary rename operation & related first

The Filings page will ultimately be part of the Transparency page,
but for now, just rename it in prep for later changes.