Added in-reply-to support to comments API

ref https://linear.app/tryghost/issue/PLG-230

- adds `in_reply_to_id` to API output
- adds `in_reply_to_snippet` to API output
  - dynamically generated from the HTML of the replied-to comment
  - excluded if the replied-to comment has been deleted or hidden
- adds `commentSnippet` to `@tryghost/html-to-plaintext`
  - skips anchor tag URLs as they won't be useful for snippet purposes
  - skips blockquotes so the snippet is more likely to contain the unique content of the replied-to comment when it's quoting a previous comment
  - returns a single line (no newline chars)
- allows setting `in_reply_to_id` when creating comments
  - id must reference a reply with the same parent
  - id must reference a published comment
- adds email notification for the original reply author when their comment is replied to

Author: kevinansfield

packages/html-to-plaintext/lib/html-to-plaintext.js

@@ -35,6 +35,7 @@ const baseSettings = {
 let excerptConverter;
 let emailConverter;
 let commentConverter;
+let commentSnippetConverter;
 
 const loadConverters = () => {
     if (excerptConverter && emailConverter) {
@@ -78,9 +79,18 @@ const loadConverters = () => {
         ]
     });
 
+    const commentSnippetSettings = mergeSettings({
+        preserveNewlines: false,
+        ignoreHref: true,
+        selectors: [
+            {selector: 'blockquote', format: 'skip'}
+        ]
+    });
+
     excerptConverter = compile(excerptSettings);
     emailConverter = compile(emailSettings);
     commentConverter = compile(commentSettings);
+    commentSnippetConverter = compile(commentSnippetSettings);
 };
 
 module.exports.excerpt = (html) => {
@@ -100,3 +110,11 @@ module.exports.comment = (html) => {
 
     return commentConverter(html);
 };
+
+module.exports.commentSnippet = (html) => {
+    loadConverters();
+
+    return commentSnippetConverter(html)
+        .replace(/\n/g, ' ')
+        .replace(/\s+/g, ' ');
+};

packages/html-to-plaintext/test/html-to-plaintext.test.js

@@ -87,4 +87,28 @@ describe('Html to Plaintext', function () {
             assert.equal(excerpt, expected);
         });
     });
+
+    describe('commentSnippet converter', function () {
+        function testConverter({input, expected}) {
+            return () => {
+                const output = htmlToPlaintext.commentSnippet(input);
+                assert.equal(output, expected);
+            };
+        }
+
+        it('skips href urls', testConverter({
+            input: '<a href="https://mysite.com">A snippet from a post template</a>',
+            expected: 'A snippet from a post template'
+        }));
+
+        it('skips blockquotes', testConverter({
+            input: '<blockquote>Previous comment quote</blockquote><p>And the new comment text</p>',
+            expected: 'And the new comment text'
+        }));
+
+        it('returns a single line', testConverter({
+            input: '<p>First paragraph.</p><p>Second paragraph.</p>',
+            expected: 'First paragraph. Second paragraph.'
+        }));
+    });
 });