Improve styling of link hierarchy

Minor but welcome change to how link hierarchies are styled that no longer requires the template to manually add <b> tags to the last link in the hierarchy. Now we just target it with :last-child, which is much smarter. Consequently, we can remove all those manual bolds from the template; all that's needed is the .link-hierarchy class on the .pair.

Relates to https://code.librehq.com/ots/clients/lfc/torque-tracker/-/issues/98