put conversation search behind feature flag

Signal/src/ViewControllers/ThreadSettings/OWSConversationSettingsViewController.m

@@ -371,7 +371,7 @@ const CGFloat kIconViewLength = 24;
                                  [weakSelf showMediaGallery];
                              }]];
 
-    // TODO icon
+    if (SSKFeatureFlags.conversationSearch) {
         [mainSection addItem:[OWSTableItem
                                  itemWithCustomCellBlock:^{
                                      NSString *title = NSLocalizedString(@"CONVERSATION_SETTINGS_SEARCH",
@@ -386,6 +386,7 @@ const CGFloat kIconViewLength = 24;
                                  actionBlock:^{
                                      [weakSelf tappedConversationSearch];
                                  }]];
+    }
 
     if (!isNoteToSelf && !self.isGroupThread && self.thread.hasSafetyNumbers) {
         [mainSection

SignalServiceKit/src/Util/FeatureFlags.swift

@@ -9,6 +9,11 @@ import Foundation
 @objc(SSKFeatureFlags)
 public class FeatureFlags: NSObject {
 
+    @objc
+    public static var conversationSearch: Bool {
+        return false
+    }
+
     /// iOS has long supported sending oversized text as a sidecar attachment. The other clients
     /// simply displayed it as a text attachment. As part of the new cross-client long-text feature,
     /// we want to be able to display long text with attachments as well. Existing iOS clients