Support horizontal rules in Classic editor

Ref T727

less/prose-editor.less

@@ -273,16 +273,17 @@ li.ProseMirror-selectednode:after {
 /* Add space around the hr to make clicking it easier */
 
 .ProseMirror-example-setup-style hr {
-	padding: 2px 10px;
+	padding: 4px 10px;
 	border: none;
 	margin: 1em 0;
+	background: initial;
 }
 
 .ProseMirror-example-setup-style hr:after {
 	content: "";
 	display: block;
 	height: 1px;
-	background-color: silver;
+	background-color: #ccc;
 	line-height: 2px;
 }
 

prose/markdownParser.js

@@ -25,7 +25,7 @@ export const writeFreelyMarkdownParser = new MarkdownParser(
       getAttrs: (tok) => ({ params: tok.info || "" }),
       noCloseToken: true,
     },
-    // hr: { node: "horizontal_rule" },
+    hr: { node: "horizontal_rule" },
     image: {
       node: "image",
       getAttrs: (tok) => ({

prose/markdownSerializer.js

@@ -47,6 +47,10 @@ export const writeFreelyMarkdownSerializer = new MarkdownSerializer(
       state.renderInline(node);
       state.closeBlock(node);
     },
+    horizontal_rule: function horizontal_rule(state, node) {
+      state.write(node.attrs.markup || "---");
+      state.closeBlock(node);
+    },
     bullet_list(state, node) {
       node.attrs.tight = true;
       state.renderList(node, "  ", () => `${node.attrs.bullet || "*"} `);

prose/prose.js

@@ -104,7 +104,7 @@ class ProseMirrorView {
   }
 
   get content() {
-    return defaultMarkdownSerializer.serialize(this.view.state.doc);
+    return writeFreelyMarkdownSerializer.serialize(this.view.state.doc);
   }
   focus() {
     this.view.focus();

prose/schema.js

@@ -4,7 +4,6 @@ import { Schema } from "prosemirror-model";
 export const writeFreelySchema = new Schema({
   nodes: schema.spec.nodes
     .remove("blockquote")
-    .remove("horizontal_rule")
     .addToEnd("readmore", {
       inline: false,
       content: "",