Add markdown alert parsing, fix buffer issue when switching files (#988)

Adds the GitHub alert syntax parsing to the markdown element, fixes an
issue where the file edit buffer was not getting unset when the file
path changed, continues my crusade on star imports

Author: esimkowitz

SECURITY.md

@@ -2,4 +2,4 @@
 
 To report vulnerabilities or security concerns, please email us at: [[email protected]](mailto:[email protected])
 
-** Please do not report security vulnerabilities through public github issues. **
+**Please do not report security vulnerabilities through public github issues.**

frontend/app/element/markdown.tsx

@@ -17,6 +17,7 @@ import rehypeSanitize, { defaultSchema } from "rehype-sanitize";
 import rehypeSlug from "rehype-slug";
 import RemarkFlexibleToc, { TocItem } from "remark-flexible-toc";
 import remarkGfm from "remark-gfm";
+import { remarkAlert } from "remark-github-blockquote-alert";
 import { openLink } from "../store/global";
 import { IconButton } from "./iconbutton";
 import "./markdown.less";
@@ -258,7 +259,7 @@ const Markdown = ({
                 options={{ scrollbars: { autoHide: "leave" } }}
             >
                 <ReactMarkdown
-                    remarkPlugins={[remarkGfm, [RemarkFlexibleToc, { tocRef: tocRef.current }]]}
+                    remarkPlugins={[remarkGfm, remarkAlert, [RemarkFlexibleToc, { tocRef: tocRef.current }]]}
                     rehypePlugins={[
                         rehypeRaw,
                         rehypeHighlight,