RCFilters: Redo the way spinners and ready/loading states are managed
The mw-rcfilters-ui-ready class was being set on no fewer than 4 elements,
all with different meanings, and some with no relevant styling at all.
It also did not distinguish between "the UI is initializing" and
"we are loading new results".
Remove mw-rcfilters-ui-ready completely (and remove some code that added
or removed it pointlessly), and create two new CSS classes on the <body>.
mw-rcfilters-ui-initialized is added when the UI has initialized, and
is used to hide the old UI and display a spinner while the UI loads.
mw-rcfilters-ui-loading is added when we start loading new results
and removed when they arrive; it's used to grey out the results
area and add a spinner.
Bonus: make the spinner appear in a different place depending on why
it's being shown: when initializing, it's shown in the place where the
UI will soon appear; when loading, it's shown near the top of the
changes list.
Change-Id: Ib5c8a36654ba44880823fdade8cad52ffa59ed69