Various fixes to scrollable lists and media gallery (#9501)

* Always use the scrollable class for the ScrollList component

Fixes #9499 as well as empty timelines' scrollbar “blinking”
between loading state and empty state.

* Do not display empty message when the list is known to have more elements

Fixes #9500

* Fix LoadMore usage in account media gallery

(Even though that codepath is probably never actually used…)

* Make account media gallery more consistent with account timeline

Fixes #9498

Display “load more” more consistently, add a loading indicator on first load.

* Fix “load more” button when no data has been fetched