Scroll to Bottom: Hide button on Tumblr Patio

[marcustyphoon]

Concept

Scroll to Bottom neither works nor makes sense in the "Tumblr Patio" feature preview, and its button gets in the way of the UI.

There isn't a particularly easy way to hide the button based on the current page, to be honest, since the container we're attaching the button to doesn't get affected by navigation... so this might be a use case for #1294.

[stale[bot]]

This issue is unconfirmed, and has been labelled as stale due to inactivity. It will be closed automatically if no further activity occurs.

A project maintainer can mark an issue as confirmed by adding the help wanted label, the wontfix label, or an assignee.

[marcustyphoon]

This should also exclude elements in patio columns from knightRiderLoaderSelector; right now the scroll to bottom button stays lit if you switch to patio while it's enabled, and will continue to be enabled if you then navigate elsewhere. (Very minor, of course.)