Add how many rooms you have in common to the user detail dialog

Eventually this will be expanded into an actual list you can look
through, but this can at least give you an idea the
number of rooms this user shares with you. If the user doesn't share any
rooms with you (e.g. they left) then the label is hidden.

src/qml/UserDetailDialog.qml

@@ -56,6 +56,8 @@ Kirigami.Dialog {
             ColumnLayout {
                 Layout.fillWidth: true
 
+                spacing: 0
+
                 Kirigami.Heading {
                     level: 1
                     Layout.fillWidth: true
@@ -71,6 +73,19 @@ Kirigami.Dialog {
                     textFormat: TextEdit.PlainText
                     text: root.user.id
                 }
+
+                QQC2.Label {
+                    property CommonRoomsModel model: CommonRoomsModel {
+                        connection: root.connection
+                        userId: root.user.id
+                    }
+
+                    text: i18ncp("@info", "One mutual room", "%1 mutual rooms", model.count)
+                    color: Kirigami.Theme.disabledTextColor
+                    visible: model.count > 0
+
+                    Layout.topMargin: Kirigami.Units.smallSpacing
+                }
             }
             QQC2.AbstractButton {
                 Layout.minimumHeight: avatar.height * 0.75