Allow teachers to set an option to launch assignments with the sidebar closed

This comes out of a feature request [via our support inbox](https://hypothesis.zendesk.com/agent/tickets/10809).

Teachers might want to set an option to launch assignments with the Hypothesis sidebar closed (right now the default is open). This user also wants highlights to be off by default. The user story from the linked ticket:

> I want to start reading without any visual “distractions”, and then enable Hypothesis later, as opposed to always closing Hypothesis [and turning off highlights] first.

From [our discussion in Slack](https://hypothes-is.slack.com/archives/CLQUUEVMY/p1592579344110000) the team agreed that it would be sensible to present "sidebar closed by default" as an option but there were questions about setting highlights off by default, mainly:

- Will students think that no discussion has happened over a text that has in fact been annotated?
- If such a toggle exists, what's the most sensible default?