Enable opening settings while in Reader mode

[tnajdek]

This would require a change in zotero/reader so that, for the web build, we expose a UI icon that sends a message to the web library to open a settings window.

We could then customise the settings so that certain options only appear while in reader mode (e.g. "Use dark mode for content"). Additionally, we could expose options currently unavailable in the web library, such as switching between scrolled and paginated mode for EPUBs (which would solve #532).