[EC-259] Added Account Switching to Share extension on iOS (#1971)

* EC-259 Added Account switching on share extension on iOS, also improved performance for this and exception handling

* EC-259 code formatting

* EC-259 Added account switching to Share extension Send view

* EC-259 Fixed navigation on share extension when a forms page is already presented

* EC-259 Fix send text UI update when going from the iOS extension

* EC-259 Improved DateTimeViewModel with helper property to easily setup date and time at the same time and applied on usage

src/App/Controls/AvatarImageSourcePool.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Collections.Concurrent;
+
+namespace Bit.App.Controls
+{
+    public interface IAvatarImageSourcePool
+    {
+        AvatarImageSource GetOrCreateAvatar(string name, string email);
+    }
+
+    public class AvatarImageSourcePool : IAvatarImageSourcePool
+    {
+        private readonly ConcurrentDictionary<string, AvatarImageSource> _cache = new ConcurrentDictionary<string, AvatarImageSource>();
+
+        public AvatarImageSource GetOrCreateAvatar(string name, string email)
+        {
+            var key = $"{name}{email}";
+            if (!_cache.TryGetValue(key, out var avatar))
+            {
+                avatar = new AvatarImageSource(name, email);
+                if (!_cache.TryAdd(key, avatar)
+                    &&
+                    !_cache.TryGetValue(key, out avatar)) // If add fails another thread created the avatar in between the first try get and the try add.
+                {
+                    // if add and get after fails, then something wrong is going on with this method.
+                    throw new InvalidOperationException("Something is wrong creating the avatar image");
+                }
+            }
+            return avatar;
+        }
+    }
+}
+