Do avatar placeholder in SwiftUI, Add AvatarViewModel, Add dependency injection for MxMediaManager dependency.

Riot/Modules/Room/NotificationSettings/SwiftUI/FormItemButtonStyle.swift

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+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Foundation
+import SwiftUI
+
+@available(iOS 14.0, *)
+struct FormItemButtonStyle: ButtonStyle {
+    @Environment(\.theme) var theme: Theme
+    func makeBody(configuration: Self.Configuration) -> some View {
+        configuration.label
+            .background(configuration.isPressed ? Color(theme.selectedBackgroundColor) : Color(theme.backgroundColor))
+            .foregroundColor(Color(theme.textPrimaryColor))
+            .font(Font(theme.fonts.body))
+    }
+}