Support blockquotes in Classic editor

Ref T727

less/prose-editor.less

@@ -293,7 +293,8 @@ li.ProseMirror-selectednode:after {
 
 .ProseMirror blockquote {
 	padding-left: 1em;
-	border-left: 3px solid #eee;
+	border-left: 4px solid #ddd;
+	color: #767676;
 	margin-left: 0;
 	margin-right: 0;
 }

prose/markdownParser.js

@@ -7,7 +7,7 @@ export const writeFreelyMarkdownParser = new MarkdownParser(
   writeFreelySchema,
   markdownit("commonmark", { html: true }),
   {
-    // blockquote: { block: "blockquote" },
+    blockquote: { block: "blockquote" },
     paragraph: { block: "paragraph" },
     list_item: { block: "list_item" },
     bullet_list: { block: "bullet_list" },

prose/markdownSerializer.js

@@ -32,9 +32,9 @@ export const writeFreelyMarkdownSerializer = new MarkdownSerializer(
       state.write("<!--more-->\n");
       state.closeBlock(node);
     },
-    // blockquote(state, node) {
+    blockquote(state, node) {
-    //   state.wrapBlock("> ", undefined, node, () => state.renderContent(node));
+      state.wrapBlock("> ", null, node, () => state.renderContent(node));
-    // },
+    },
     code_block(state, node) {
       state.write(`\`\`\`${node.attrs.params || ""}\n`);
       state.text(node.textContent, false);

prose/schema.js

@@ -3,7 +3,6 @@ import { Schema } from "prosemirror-model";
 
 export const writeFreelySchema = new Schema({
   nodes: schema.spec.nodes
-    .remove("blockquote")
     .addToEnd("readmore", {
       inline: false,
       content: "",