Implement MSC 4228 Search Redirection

See https://github.com/matrix-org/matrix-spec-proposals/pull/4228 for details.
Since this is tricky to test without server-side support, I have added a basic implementation
to the mock server in appiumtests/login-server.py

1. Start appiumtests/login-server.py
2. Start neochat with "--test --ignore-ssl-errors" options
3. Open "Explore Rooms"
4. Search for the exact string "forbidden"
5. See new error message provided by server

src/qml/SearchPage.qml

@@ -80,6 +80,17 @@ Kirigami.ScrollablePage {
      */
     property bool showSearchButton: true
 
+    /**
+     * @brief Message to be shown in a custom placeholder.
+     * The custom placeholder will be shown if the text is not empty
+     */
+    property alias customPlaceholderText: customPlaceholder.text
+
+    /**
+     * @brief icon for the custom placeholder
+     */
+    property string customPlaceholderIcon: ""
+
     /**
      * @brief Force the search field to be focussed.
      */
@@ -167,18 +178,25 @@ Kirigami.ScrollablePage {
         Kirigami.PlaceholderMessage {
             id: noSearchMessage
             anchors.centerIn: parent
-            visible: searchField.text.length === 0 && listView.count === 0
+            visible: searchField.text.length === 0 && listView.count === 0 && !root.showCustomPlaceholder && customPlaceholder.text.length === 0
         }
 
         Kirigami.PlaceholderMessage {
             id: noResultMessage
             anchors.centerIn: parent
-            visible: searchField.text.length > 0 && listView.count === 0 && !root.model.searching
+            visible: searchField.text.length > 0 && listView.count === 0 && !root.model.searching && customPlaceholder.text.length === 0
+        }
+
+        Kirigami.PlaceholderMessage {
+            id: customPlaceholder
+            anchors.centerIn: parent
+            visible: searchField.text.length > 0 && listView.count === 0 && !root.model.searching && text.length > 0
+            icon.name: root.customPlaceholderIcon
         }
 
         Kirigami.LoadingPlaceholder {
             anchors.centerIn: parent
-            visible: searchField.text.length > 0 && listView.count === 0 && root.model.searching
+            visible: searchField.text.length > 0 && listView.count === 0 && root.model.searching && customPlaceholder.text.length === 0
         }
 
         Keys.onUpPressed: {