Clarify the difference between tracking and adding to a list

[GPHemsley]

I think they are the same thing? But some pages have two buttons – one for "Track this" and one for "Add to list" – while others only have the latter. And still others have neither. (See, e.g., #259.)

In addition, certain topics (stuff like "Recent Legislative Activity") present a completely different UI for adding to lists, which only allows you to add it to one list at a time, and does not give any indicator as to whether it is already in an existing list. However, that particular UI does give you the added benefit of identifying examples of what would be tracked, which is not available in the general list UI.