For the case where cross-signing is trusted on device but secret storage does
not exist, we were showing "verify this device", which is not the best match
from the existing toasts. This tunes the checks to instead show "set up
encryption" which is at least a bit closer.
Part of https://github.com/vector-im/element-web/issues/14970
If the Secure Backup required mode is set the client `.well-known` file, then
this will ensure that everyone already inside the app is required to complete
setup matching that policy.
Fixes https://github.com/vector-im/element-web/issues/14954
This improves the experience of going through secret storage setup / reset flows
by avoiding intermittent toasts that appear and disappear in the middle of the
operation.