Improve UX when loading new content

[jacobwegner]

Currently:

• Some widgets use loading indicators that replace the entire widget
• Some widgets have no indicator and appear laggy

Desired:

• For opened sidebar widgets, have a consistent loading icon that replaces the chevron
• While loading action is in flight, suggest changing opacity of entire widget
• For the main widget, we should "overlay" some kind of loading indicator, but also change opacity
• Consider slight animation?

[jacobwegner]

@paltman and I had a discussion about this on a call...we'll likely want to have some interaction with the widget "contract" to reset scroll state too

[jacobwegner]

as another data point, when paginating on v1, we've disabled next / prev until loading is done

https://scaife-perseus-org-dev.herokuapp.com/reader/urn:cts:greekLit:tlg0012.tlg001.perseus-grc2:1.57-1.63

in v2, you can still click next / prev while loading, which effectively is a no op, but it feels weird:

https://explorehomer.scaife-viewer.org/reader?urn=urn%3Acts%3AgreekLit%3Atlg0012.tlg001.perseus-grc2%3A1.29-1.35