Implement the save action when previewing media. (#3630)

* Implement the save action on the media preview.

* Update Compound and use the correct icon.

Also fixes an icon that has been renamed.

* Update the add to photo library usage description to match the designs.

* PR comments.

ElementX/Sources/Mocks/PhotoLibraryManagerMock.swift

@@ -0,0 +1,21 @@
+//
+// Copyright 2024 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only
+// Please see LICENSE in the repository root for full details.
+//
+
+import Foundation
+
+extension PhotoLibraryManagerMock {
+    struct Configuration {
+        var authorizationDenied = false
+    }
+    
+    // swiftlint:disable:next cyclomatic_complexity
+    convenience init(_ configuration: Configuration) {
+        self.init()
+        
+        addResourceAtReturnValue = configuration.authorizationDenied ? .failure(PhotoLibraryManagerError.notAuthorized) : .success(())
+    }
+}