BTCPay Extensions Part 2 (#2001)

* BTCPay Extensions Part 2

This PR cleans up the extension system a bit in that:
 * It renames the test extension to a more uniform name
 * Allows yo uto have system extensions, which are extensions but bundled by default with the release (and cannot be removed)
 * Adds a tool to help you generate an extension package from a csproj
 * Refactors the UI extension points to a view component
 * Moves some more interfaces to the Abstractions csproj

* Rename to plugins

BTCPayServer.Abstractions/Models/StatusMessageModel.cs

@@ -0,0 +1,50 @@
+using System;
+
+namespace BTCPayServer.Models
+{
+    public class StatusMessageModel
+    {
+        public StatusMessageModel()
+        {
+        }
+        public string Message { get; set; }
+        public string Html { get; set; }
+        public StatusSeverity Severity { get; set; }
+        public bool AllowDismiss { get; set; } = true;
+
+        public string SeverityCSS => ToString(Severity);
+
+        private void ParseNonJsonStatus(string s)
+        {
+            Message = s;
+            Severity = s.StartsWith("Error", StringComparison.InvariantCultureIgnoreCase)
+                ? StatusSeverity.Error
+                : StatusSeverity.Success;
+        }
+
+        public static string ToString(StatusSeverity severity)
+        {
+            switch (severity)
+            {
+                case StatusSeverity.Info:
+                    return "info";
+                case StatusSeverity.Error:
+                    return "danger";
+                case StatusSeverity.Success:
+                    return "success";
+                case StatusSeverity.Warning:
+                    return "warning";
+                default:
+                    throw new ArgumentOutOfRangeException();
+            }
+        }
+
+        public enum StatusSeverity
+        {
+            Info,
+            Error,
+            Success,
+            Warning
+        }
+    }
+}