Add Hypothesis watermark to the Sidebar

Conversation started here:
https://hypothes-is.slack.com/archives/C1EQELNMS/p1583186190016700

Currently when Hypothesis is embedded in a site, there is no indication or branding that suggests that Hypothesis is a separate entity from the site itself. A phrase like “Annotations powered by Hypothesis” in the sidebar would help users understand that we can be embedded in other sites as well.

There is also the idea that we should add a logo where we're powering the backend, but the site has their own interface for annotations (like Science in the Classroom).

Some ideas from the conversation:
- watermark in the Sidebar
- watermark in the Menu
- a lightweight widget that we use in the sidebar but that external uses (like SitC) could also embed

Please add other approaches.