Redesign incoming real-time annotation signal

## Feature Request Form
### Problem you are trying to address with this feature

The red icon at the top of the sidebar indicating that someone has recently made an annotation on that page is not obvious to users.

Example: https://twitter.com/BMBOD/status/795361337463468033

### Your solution

We should:
- Redesign the indicator, to be more explicit, or
- Find a way to address the original issue, which was that the sidebar jumped around when there were incoming annotations. Particularly when there were many in a short period of time, and someone might be in the middle of drafting their own annotation.