Use correct icon for "Add Caption" vs. "Has Caption"

Signal/Images.xcassets/image_editor_add_caption.imageset/Contents.json

@@ -0,0 +1,23 @@
+{
+  "images" : [
+    {
+      "idiom" : "universal",
+      "filename" : "add-caption-28@1x.png",
+      "scale" : "1x"
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "add-caption-28@2x.png",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "add-caption-28@3x.png",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}

Signal/Images.xcassets/image_editor_caption.imageset/Contents.json

@@ -2,17 +2,17 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "add-caption-28@1x.png",
+      "filename" : "caption-24@1x.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
-      "filename" : "add-caption-28@2x.png",
+      "filename" : "caption-24@2x.png",
       "scale" : "2x"
     },
     {
       "idiom" : "universal",
-      "filename" : "add-caption-28@3x.png",
+      "filename" : "caption-24@3x.png",
       "scale" : "3x"
     }
   ],

SignalMessaging/ViewControllers/AttachmentApproval/AttachmentApprovalViewController.swift

@@ -278,9 +278,9 @@ public class AttachmentApprovalViewController: UIPageViewController, UIPageViewC
                 shouldShowCaptionUI = true
             }
             if shouldShowCaptionUI {
-                let captionButton = navigationBarButton(imageName: "image_editor_caption",
-                                                        selector: #selector(didTapCaption(sender:)))
-                navigationBarItems.append(captionButton)
+                let addCaptionButton = navigationBarButton(imageName: "image_editor_add_caption",
+                                                           selector: #selector(didTapCaption(sender:)))
+                navigationBarItems.append(addCaptionButton)
             }
         }