resolves #189 add support for align=left and align=right on block image

* float-group establishes a block formatting context (without affecting margin collapsing)
* float-gap adds side and bottom margin to a floating element

resolves #187 switch from deprecated typeface- packages to fontsource packages to retrieve fonts

resolves #187 switch from deprecated typeface- packages to fontsource packages to retrieve fonts; use more liberal filter

* customize scrollbars on html element
* align scrollbar track width between Firefox and Webkit-based browsers
* align scrollbar thumb between Firefox and Webkit-based browsers

resolves #184 use same format of "videoblock" as "imageblock"

Style the search box when disabled

document the site.homeUrl property in the UI model; clarify how it differs from site.url and siteRootPath

resolves #181 prevent overscroll event in scrollable region from bubbling

add search input behind a flag