Attempt to fix the flakyness seen with tests
Specifically:
```
JS 2.1.1 (Linux 0.0.0) joining a room over federation should not get stuck at a spinner FAILED
Did not find exactly one match (found: 0) for componentType:function (props, context, updater) {
```
actually meant that the room directory wasn't displayed - probably because the dispatch `view_room_directory` ended up on another tick of the event loop, meaning that the directory wasn't displayedi. The fix attempted in ths commit is to use `this._setPage` instead to view the directory. This uses `setState` to set the screen to the directory, so I'm not entirely convinced this will solve the problem (as `setState` may also end up doing things on another tick.
and
```
JS 2.1.1 (Linux 0.0.0) loading: MatrixClient rehydrated from stored credentials: shows a room view if we followed a room link FAILED
MatrixChat still not ready after 5 tries
awaitRoomView@/home/travis/build/vector-im/riot-web/test/all-tests.js:201363:90
```
was happening probably because in the handler for the `sync` event in `MatrixChat` (around line 840), there was one case in which the `ready` state may not be true (causing all 5 attempts to fail), and this case relied on `starting_room_alias_payload`. This `starting_room_alias_payload` is now redundant because of `initialScreenAfterLogin`, which was added recently.
