move category method to be shared

Signal/src/ViewControllers/MediaGalleryViewController.swift

@@ -4,12 +4,6 @@
 
 import Foundation
 
-extension String {
-    var filterForDisplay: String? {
-        return (self as NSString).filterStringForDisplay()
-    }
-}
-
 public enum GalleryDirection {
     case before, after, around
 }

SignalMessaging/categories/String+OWS.swift

@@ -14,6 +14,10 @@ public extension String {
         return (self as NSString).rtlSafeAppend(string)
     }
 
+    var filterForDisplay: String? {
+        return (self as NSString).filterStringForDisplay()
+    }
+
     // Truncates string to be less than or equal to byteCount, while ensuring we never truncate partial characters for multibyte characters.
     func truncated(toByteCount byteCount: UInt) -> String? {
         var lowerBoundCharCount = 0