add reading progress popovers

[riastrad]

Closes #274

This updates the currently reading sections of the website to trigger a popover with some additional information. Rather than just have them be jumplinks to external sites.

Still to do:

• Fix spacing issues with popover divs
• Fix popover styling
• Add a ::backdrop to popovers

Won't do:

• Incorporate popover solution on bookshelf's 'currently reading' section

[riastrad]

Here's the current state of the home page. I'm happy with it so far. Very happy that I can get to this point without adding new javascript to the home page.

Only issues remaining for home page are some spacing issues caused by the parser inserting unexpected <p> elements. Ideally I'd find a way to prevent those elements from ending up in the final build...

[riastrad]

💡 idea for a blogpost: <progress> is a nightmare

[riastrad]

Style is in a good place so I'm going to stop fiddling. Need to fix the spacing before this can merge though.

Not entirely convinced I should wrestle this onto the bookshelf page yet...

[riastrad]

Pretty sure I've nailed this down for the home page:

desktop

mobile screens

[riastrad]

I'm going to run with this and skip the bookshelf updates for now. Can revisit in future if it feels like they're needed to feel consistent.

[riastrad]

Live and looking sharp - here it is on a proper mobile device.