Avoid using accessSecretStorage to create 4S (#30244)

* remove resetCrossSigning flag, which is no longer in use

* drop unnecessary check for cross-signing

The only place where verifyUser is called already checks that cross-signing is
set up.  (The function name is also incorrect, since it checks for the
cross-signing key, and not for 4S.)

* avoid calling accessSecretStorage to set up cross-signing or 4S

Send the user to the Encryption settings tab instead

* only create secret storage when specifically asked to

* deprecate using accessSecretStorage to create new 4S

* also remove the obsolete snapshot

* add tests

* Tweak comment

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

res/css/views/dialogs/security/_CreateKeyBackupDialog.pcss

@@ -1,73 +0,0 @@
-/*
-Copyright 2018-2024 New Vector Ltd.
-
-SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only OR LicenseRef-Element-Commercial
-Please see LICENSE files in the repository root for full details.
-*/
-
-.mx_CreateKeyBackupDialog .mx_Dialog_title {
-    /* TODO: Consider setting this for all dialog titles. */
-    margin-bottom: 1em;
-}
-
-.mx_CreateKeyBackupDialog_primaryContainer {
-    /* FIXME: plinth colour in new theme(s). background-color: $accent; */
-    padding: 20px;
-}
-
-.mx_CreateKeyBackupDialog_primaryContainer::after {
-    content: "";
-    clear: both;
-    display: block;
-}
-
-.mx_CreateKeyBackupDialog_passPhraseContainer {
-    display: flex;
-    align-items: flex-start;
-}
-
-.mx_CreateKeyBackupDialog_passPhraseInput {
-    flex: none;
-    width: 250px;
-    border: 1px solid $accent;
-    border-radius: 5px;
-    padding: 10px;
-    margin-bottom: 1em;
-}
-
-.mx_CreateKeyBackupDialog_passPhraseMatch {
-    margin-left: 20px;
-}
-
-.mx_CreateKeyBackupDialog_recoveryKeyHeader {
-    margin-bottom: 1em;
-}
-
-.mx_CreateKeyBackupDialog_recoveryKeyContainer {
-    display: flex;
-}
-
-.mx_CreateKeyBackupDialog_recoveryKey {
-    width: 262px;
-    padding: 20px;
-    color: $info-plinth-fg-color;
-    background-color: $info-plinth-bg-color;
-    margin-right: 12px;
-}
-
-.mx_CreateKeyBackupDialog_recoveryKeyButtons {
-    flex: 1;
-    display: flex;
-    align-items: center;
-}
-
-.mx_CreateKeyBackupDialog_recoveryKeyButtons button {
-    flex: 1;
-    white-space: nowrap;
-}
-
-.mx_CreateKeyBackupDialog {
-    details .mx_AccessibleButton {
-        margin: 1em 0; /* emulate paragraph spacing because we can't put this button in a paragraph due to HTML rules */
-    }
-}