Prevent users from trying to search in encrypted rooms

I forgot that this has to be handled client-side (duh, we're the only
one who can read the messages!) This has already tripped up at least one
user, and probably more. So until we support that, our UI shouldn't lie
to you.

src/roominfo/RoomSearchPage.qml

@@ -30,6 +30,9 @@ SearchPage {
      */
     property string senderId
 
+    // This requires client-side search we don't implement yet
+    readonly property bool canSearch: !room.usesEncryption
+
     title: i18nc("@action:title", "Search Messages")
 
     model: SearchModel {
@@ -45,6 +48,9 @@ SearchPage {
     searchFieldPlaceholder: i18n("Find messages…")
     noSearchPlaceholderMessage: i18n("Enter text to start searching")
     noResultPlaceholderMessage: i18n("No messages found")
+    customPlaceholderIcon: "lock-symbolic"
+    customPlaceholderText: !canSearch ? i18n("Cannot search in encrypted rooms") : ""
+    enableSearch: canSearch
 
     listVerticalLayoutDirection: ListView.BottomToTop