Give application actors in feed reasonable icons

Summary:
Ref T8952. Currently, when an application (most commonly Herald, but sometimes Drydock, Diffusion, etc) publishes a feed story, we get an empty grey box for it in feed.

Instead, give the story a little application icon kind of "profile picture"-like thing.

Test Plan:
Here's how it looks:

Feel free to tweak/counter-diff.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8952

Differential Revision: https://secure.phabricator.com/D15773