Aggregate activity data ("Boosted/Liked by")

Thank your for Phanpy; it's truly excellent in many ways! (In fact, it's become sort of a reference web application for me, including for technical discussions.)

**Is your feature request related to a problem? Please describe.**

The "Boosted/Liked by" dialog currently only includes activity from the current Mastodon instance, which might be incomplete and thus misleading. As I understand it, retrieving authoritative data requires switching to the respective post's instance before opening that dialog.

(Aside: This requires interacting with two similar, yet different menus - which is less than intuitive and perhaps worth a separate issue?)

**Describe the solution you'd like**

While the underlying mechanisms are inherent in Mastodon's federation, the client (i.e. Phanpy) could automatically aggregate activity data to improve user experience (i.e. save clicks and report consistent data).

I'm not sure whether this should be optional though; do people care about activity only on their own instance?

(Apologies if this has already been discussed; I couldn't find any existing issues. **Update:** The discussion in #168 seems at least related.)