[Linked fields] Add Linked Field as a custom field type (#1563)

* Add linked fields support

* Fix style, don't show linked field if Secure Note

* Finish basic linked fields for Login

* Use Field.LinkedId to store linked field info

* Reset Linked Custom Fields if cipherType changes

* Refactor to use ItemView class

* Use enum for LinkedId

* Detect if no linkedFieldOptions

src/Core/Models/View/ItemView.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+using Bit.Core.Enums;
+
+namespace Bit.Core.Models.View
+{
+    public abstract class ItemView : View
+    {
+        public ItemView() { }
+
+        public abstract string SubTitle { get; }
+
+        public abstract List<KeyValuePair<string, LinkedIdType>> LinkedFieldOptions { get; }
+    }
+}