Remove "attachment" from gallery terminology

assets/trix/stylesheets/content.scss

@@ -106,8 +106,8 @@
       max-width: 33%;
     }
 
-    &[data-trix-attachment-count="2"],
-    &[data-trix-attachment-count="4"] {
+    &[data-trix-gallery-size="2"],
+    &[data-trix-gallery-size="4"] {
       .attachment-container {
         flex-basis: 50%;
         max-width: 50%;

src/trix/config/css.coffee

@@ -6,4 +6,4 @@ Trix.config.css =
   attachmentCaption: "attachment__caption"
   attachmentCaptionEditor: "attachment__caption-editor"
   attachmentContainer: "attachment-container"
-  attachmentGallery: "attachment-gallery"
+  gallery: "attachment-gallery"

src/trix/views/block_view.coffee

@@ -33,9 +33,9 @@ class Trix.BlockView extends Trix.ObjectView
     makeElement do =>
       if attribute is "gallery"
         tagName: "div"
-        className: css.attachmentGallery
+        className: css.gallery
         attributes: role: "group"
-        data: trixAttachmentCount: @block.getBlockBreakPosition()
+        data: trixGallerySize: @block.getBlockBreakPosition()
       else
         {tagName} = getBlockConfig(attribute)
         {tagName}