Show the platform from which a post is sent.

**The underlying problem**

The Fediverse is not just Mastodon. But currently it is difficult to find out the platform from which a post was originally sent, except by going to the original page of a post and then trying to find out on which software it runs (which is not always simple). For the general view on the Fediverse — and for my own curiosity — a simpler way to find out would be better.

**Idea for a solution**
Place the name or the symbol of the sender's platform at the header of each post. Or, if that would be too cluttered, put it into the tooltip that appears when one hovers over the poster's name.

No files or tests are named. Start by locating the post header and poster-name tooltip rendering, then check whether the sender platform is available in the post data. Done means showing the platform name or symbol without making the post header or tooltip unclear.

Written by the indexing model from the issue text.