Simplified Sliding Sync (#28515)

* Experimental SSS

Working branch to get SSS functional on element-web.

Requires https://github.com/matrix-org/matrix-js-sdk/pull/4400

* Adjust tests to use new behaviour

* Remove well-known proxy URL lookup; always use native

This is actually required for SSS because otherwise it would use
the proxy over native support.

* Linting

* Debug logging

* Control the race condition when swapping between rooms

* Dont' filter by space as synapse doesn't support it

* Remove SS code related to registering lists and managing ranges

- Update the spidering code to spider all the relevant lists.
- Add canonical alias to the required_state to allow room name calcs to work.

Room sort order is busted because we don't yet look at `bump_stamp`.

* User bumpStamp if it is present

* Drop initial room load from 20 per list to 10

* Half the batch size to trickle more quickly

* Prettier

* prettier on tests too

* Remove proxy URL & unused import

* Hopefully fix tests to assert what the behaviour is supposed to be

* Move the singleton to the manager tyo fix import loop

* Very well, code, I will remove you

Why were you there in the first place?

* Strip out more unused stuff

* Fix playwright test

Seems like this lack of order updating unless a room is selected
was just always a bug with both regular and non-sliding sync. I
have no idea how the test passed on develop because it won't run.

* Fix test to do maybe what it was supposed to do... possibly?

* Remove test for old pre-simplified sliding sync behaviour

* Unused import

* Remove sliding sync proxy & test

I was wrong about what this test was asserting, it was suposed
to assert that notification dots aren't shown (because SS didn't
support them somehow I guess) but they are fine in SSS so the test
is just no longer relevant.

* Remove now pointless credentials

* Remove subscription removal as SSS doesn't do that

* Update tests

* add test

* Switch to new labs flag & break if old labs flag is enabled

* Remove unused import & fix test

* Fix other test

* Remove name & description from old labs flag

as they're not displayed anywhere so not useful

* Remove old sliding sync option

by making it not a feature

* Add back unread nindicator test but inverted

and minus the bit about disabling notification which surely would have
defeated the original point anyway?

* Reinstate test for room_subscriptions

...and also make tests actually use sliding sync

* Use UserFriendlyError

* Remove empty constructor

* Remove unrelated changes

* Unused import

* Fix import

* Avoid moving import

---------

Co-authored-by: Kegan Dougal <7190048+kegsay@users.noreply.github.com>

test/test-utils/test-utils.ts

@@ -661,6 +661,7 @@ export function mkStubRoom(
         getUnreadNotificationCount: jest.fn(() => 0),
         getRoomUnreadNotificationCount: jest.fn().mockReturnValue(0),
         getVersion: jest.fn().mockReturnValue("1"),
+        getBumpStamp: jest.fn().mockReturnValue(0),
         hasMembershipState: () => false,
         isElementVideoRoom: jest.fn().mockReturnValue(false),
         isSpaceRoom: jest.fn().mockReturnValue(false),