#40: Add UserSessionStoreProtocol.

Only log out of the specific account.
Add tests for the keychain controller.
Expand test coverage.
PR comments

ElementX/Sources/Screens/Authentication/UIConstants.swift

@@ -0,0 +1,26 @@
+// 
+// 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 SwiftUI
+
+/// Standard constants used across the app's UI.
+struct UIConstants {
+    static let maxContentWidth: CGFloat = 600
+    static let maxContentHeight: CGFloat = 750
+    
+    /// The padding used between the top of the main content and the navigation bar.
+    static let topPaddingToNavigationBar: CGFloat = 16
+}