Introduce Markdown extensions (#1614)

* Introduce markdown extensions

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/components/markdown-renderer/markdown-extension/legacy-short-codes/legacy-shortcodes-markdown-extension.ts

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+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { MarkdownExtension } from '../markdown-extension'
+import type MarkdownIt from 'markdown-it'
+import { legacyPdfShortCode } from './replace-legacy-pdf-short-code'
+import { legacySlideshareShortCode } from './replace-legacy-slideshare-short-code'
+import { legacySpeakerdeckShortCode } from './replace-legacy-speakerdeck-short-code'
+
+/**
+ * Adds support for legacy shortcodes (pdf, slideshare and speakerdeck) by replacing them with anchor elements.
+ */
+export class LegacyShortcodesMarkdownExtension extends MarkdownExtension {
+  public configureMarkdownIt(markdownIt: MarkdownIt): void {
+    legacyPdfShortCode(markdownIt)
+    legacySlideshareShortCode(markdownIt)
+    legacySpeakerdeckShortCode(markdownIt)
+  }
+}

src/components/markdown-renderer/markdown-extension/legacy-short-codes/replace-legacy-pdf-short-code.ts

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+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import markdownItRegex from 'markdown-it-regex'
+import type MarkdownIt from 'markdown-it/lib'
+
+const finalRegex = /^{%pdf (.*) ?%}$/
+
+export const legacyPdfShortCode: MarkdownIt.PluginSimple = (markdownIt) => {
+  markdownItRegex(markdownIt, {
+    name: 'legacy-pdf-short-code',
+    regex: finalRegex,
+    replace: (match: string) => `<a href="${match}">${match}</a>`
+  })
+}

src/components/markdown-renderer/markdown-extension/legacy-short-codes/replace-legacy-slideshare-short-code.ts

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+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import markdownItRegex from 'markdown-it-regex'
+import type MarkdownIt from 'markdown-it/lib'
+
+const finalRegex = /^{%slideshare (\w+\/[\w-]+) ?%}$/
+
+export const legacySlideshareShortCode: MarkdownIt.PluginSimple = (markdownIt) => {
+  markdownItRegex(markdownIt, {
+    name: 'legacy-slideshare-short-code',
+    regex: finalRegex,
+    replace: (match: string) => `<a href='https://www.slideshare.net/${match}'>https://www.slideshare.net/${match}</a>`
+  })
+}

src/components/markdown-renderer/markdown-extension/legacy-short-codes/replace-legacy-speakerdeck-short-code.ts

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+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import markdownItRegex from 'markdown-it-regex'
+import type MarkdownIt from 'markdown-it/lib'
+
+const finalRegex = /^{%speakerdeck (\w+\/[\w-]+) ?%}$/
+
+export const legacySpeakerdeckShortCode: MarkdownIt.PluginSimple = (markdownIt) => {
+  markdownItRegex(markdownIt, {
+    name: 'legacy-speakerdeck-short-code',
+    regex: finalRegex,
+    replace: (match: string) => `<a href="https://speakerdeck.com/${match}">https://speakerdeck.com/${match}</a>`
+  })
+}