Add "reader view" mode

[lizadaly]

This experimental mode will leverage the printable HTML view, with the following characteristics:

• It should use the printable HTML Django templates and JS that adds annotations/elisions.
• It should not display the page through PagedJS, but instead use a standard single-column scrolling layout.
• It should be paginated at the top-level "section" level, not at the individual resource level.
• It should be possible to toggle in and out of this mode.
• It should re-use all the custom print CSS associated with the printable view that are supported by browsers (e.g. not polyfilled by PagedJS). To the extent possible this should give us high-quality printed output that just misses out on some of the book-ish affordances of PagedJS.

Some example differences from the PagedJS layout:

• Annotations will not be rendered as footnotes, but as web-friendly sidebar annotations.
• Hyperlinks will be rendered as normal underlined and active links, not as footnotes.
• Elisions can (potentially) be toggled open.

[lizadaly]

Still to do:

• Change hyperlinks rendering to normal, active links
• Ensure Matomo tracker is in the custom HTML head (or use the site-wide base.html if that doesn't interfere with PagedJS).

[lizadaly]

• Consider using the left-nav for persistent section/resource headings
• Margins are overly generous
• Small viewport layout

[lizadaly]

one thing that might be a helpful add on would be an easy way to navigate back to the toc once you're in a specific resource on the reader page