Readd legacy pdf short code markdown it plugin

cypress/integration/shortcodes.spec.ts

@@ -0,0 +1,20 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+describe('Short code', () => {
+  beforeEach(() => {
+    cy.visitTestEditor()
+  })
+
+  describe('for pdfs', () => {
+    it('renders a plain link', () => {
+      cy.codemirrorFill(`{%pdf https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf %}`)
+      cy.getMarkdownBody()
+        .find('a')
+        .should('have.attr', 'href', 'https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf')
+    })
+  })
+})

src/components/markdown-renderer/markdown-it-configurator/FullMarkdownItConfigurator.tsx

@@ -23,6 +23,7 @@ import { VimeoReplacer } from '../replace-components/vimeo/vimeo-replacer'
 import { YoutubeReplacer } from '../replace-components/youtube/youtube-replacer'
 import { BasicMarkdownItConfigurator } from './BasicMarkdownItConfigurator'
 import { quoteExtraColor } from '../markdown-it-plugins/quote-extra-color'
+import { legacyPdfShortCode } from '../regex-plugins/replace-legacy-pdf-short-code'
 
 export class FullMarkdownItConfigurator extends BasicMarkdownItConfigurator {
   constructor(
@@ -54,6 +55,7 @@ export class FullMarkdownItConfigurator extends BasicMarkdownItConfigurator {
       YoutubeReplacer.markdownItPlugin,
       VimeoReplacer.markdownItPlugin,
       GistReplacer.markdownItPlugin,
+      legacyPdfShortCode,
       legacySlideshareShortCode,
       legacySpeakerdeckShortCode,
       AsciinemaReplacer.markdownItPlugin,