This untangles cross-signing and secret storage setup into separate path that
can be invoked independently. There is no functional change with this patch, but
instead this just separates one giant monster API into two.
Part of https://github.com/vector-im/element-web/issues/13895
In order to handle auth errors (such as incorrect passwords), we need to ensure
we only try to upload cross-signing keys from within the auth flow helper
function.
This rearranges things to store that function in the builder to use it when the
actual upload happens.
The earlier commit, d3ce0cb82f, has most of the juicy details on this. In addition to d3ce's changes, we also:
* Use `TestClient` in many integration tests due to subtle behaviour changes in imports when switching to ES6. Namely the behaviour where setting the request function is less reliable in the way we did it, but `TestClient` is very reliable.
* We now use the Olm loader more often to avoid having to maintain so much duplicate code. This makes the imports slightly easier to read.
Use options.cryptoCallbacks for things that require information
from the app rather than events, since events can have zero, one
or many listeners and the emitter doesn't know how many, so if
nobody's listening then we would have just waited forever for a
response.
Also a collection of other changes like renaming 'fu' to 'firstUse'
