Convert src/SecurityManager.js to TypeScript

This includes a few small API tweaks as well, only in cases where TS revealed we
were doing something confusing or wrong.

Part of https://github.com/vector-im/element-web/issues/15350

src/@types/global.d.ts

@@ -14,6 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import "matrix-js-sdk/src/@types/global"; // load matrix-js-sdk's type extensions first
 import * as ModernizrStatic from "modernizr";
 import ContentMessages from "../ContentMessages";
 import { IMatrixClientPeg } from "../MatrixClientPeg";