feat: use cmark-gfm for rendering blog posts

[BinaryMuse]

Per #2102, this PR updates the site to use cmark-gfm (via BinaryMuse/electron-markdown) instead of hubdown for rendering blog posts (this is the only markdown rendering done directly by the site). The PR also makes a few other refactoring-style changes to the blog route handlers.

• Replaces hubdown with cmark-gfm
• Removes blog middleware and replaces it with actual routes that use a new blog lib for finding and parsing blog posts
• Modifies blog feed to use the new blog lib
• Ensures blog posts are loaded asynchronously; fixes race condition where blog routes could run with no posts loaded
• Moves blog views from views/posts to views/blog
• Changes Node version from 8 to 10

Todo:

• Add IDs to headings
• Turn headings into links
• Syntax highlighting
• Fix extra space at bottom of code listings
• Fix iframes (e.g. YouTube videos on /blog/2015-whats-new-in-electron)

[BinaryMuse]

Okay, I believe this is feature-complete when compared with the previous renderer. I still want to do some comparisons of the old and new results to make sure nothing unexpected broke.

[BinaryMuse]

@electron/wg-website This is ready for review.

I downloaded a copy of every blog post on the current site and on this PR's deployment, and generated a diff for each. The results show that the changes are either (1) whitespace changes (particularly an extra non-visible newline at the end of code blocks), (2) single/double quote changes, (3) some <li> elements no longer have their contents wrapped in <p>, and (4) the addition of autolinks.

[vhashimotoo]