Add edge/bottom background to query title banner

[iandunn]

#64 restored the query-title-banner. in some situations, though, it needs to have the edge/bottom background added instead of the local nav.

• query-title-banner should always have the edge/bottom background
• When query-title-banner isn't shown, the local nav should have the edge/bottom background
• Probably setup an @extend %header-edge-background mixin like 298bb0b did, to make the styles DRY

Could maybe just move the query title template part inside of the local header template part, so that the container is the one that always has the background. That'd be simpler, but would require hiding the banner w/ CSS on screens where it's not desired, which is a bit hacky. It could be made cleaner once WordPress/gutenberg#32939 is solved. the "all posts" part shouldn't be position: fixed, though, only the admin bar, global header, and local nav

related #67

[iandunn]

Fixed by b2ea203