Update identity reset UI (Make consistent with EX)

[andybalaam]

Fixes #29227

This adds IdentityResetDialog, which wraps IdentityResetBody in a dialog, and launches it from SetupEncryptionBody, instead of doing the reset directly there. In the new flow, only the user's cryptographic identity is reset -- recovery is no longer set up at the same time.

This is mostly work by @uhoreg , rebased and refactored a bit here.

I recommend reviewing commit-by-commit.

[richvdh]

notwithstanding my comments about getting rid of returnAfterReset, there is an easier way to do this.

1. Have ResetIdentityDialog's onFinished prop take an optional boolean success argument. Where we run that callback from onResetWrapper, set the argument to true.
2. Use the finished promise returned by createDialog to figure out if we successfully did the reset:

      const handle = Modal.createDialog(
          ResetIdentityDialog,
          /* props= */ {
              variant: "confirm",
          },
      );
      handle.finished.then(([success]) => {
          if (success) {
              // The user completed the reset process - close this dialog
              this.props.onFinished();
              store.done();
          } else {
              // The user clicked away - go back a step
              store.returnAfterReset();
          }
      }); 

[richvdh]

This looks basically fine now but I think there's a bit of cruft left that we can clean up.

[richvdh]

A few more suggestions which might help make this clearer. The Modal API is unintuitive and very poorly documented 😢