Refactor of RYScaffold to use TopAppBar with scrolling behaviour

[yajaru]

This is a small refactor of the RYScaffold to use a TopAppBar with nested scrolling instead of an in-content header.

This also moves the Switch Account action from being on the Feeds page header text on-click to an actual dedicated button. I think this is more discoverable b/c it is not obvious how to switch accounts from the Feeds page if you don't already know about this feature.

Jump to top is also preserved for the browsing pages via an on-click callback on RYScaffold's new TopAppBar. This is basically what it was before, just move around a little.

Note that there is some goofyness around pages that are smaller that the viewport. Ideally I'd like them to not be scrollable at all but, I was not able to figure out how to do that elegantly. I could always pass down a boolean flag to RYScaffold to turn the scrolling off but, that feels nasty. If you have any ideas, let me know.

Before

After

[JunkFood02]

use a TopAppBar with nested scrolling instead of an in-content header

Why?

[JunkFood02]

The LargeTopAppBar component doesn't offer the flexibility we need. We can't customize the text styles for the large and small titles, the content padding, and other aspects. Additionally, using nestedScroll might lead to unexpected issues like what you've mentioned. I've implemented a workaround in #896. Please take a look and let me know if you're happy with the result