Added GitHub PR liquid tag support

[Glennmen]

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Documentation Update

Description

Add support for Github pull request via liquid tags in markdown.
{% github https://github.com/thepracticaldev/dev.to/pull/1633 %}

My first time setting up a Ruby environment and actually developing in Ruby, so let me know if I should have done things differently.

I also fixed a couple of pre-commit errors, strange that the thrown errors where from parts that I didn't edit. So I assume that a lot of people skip the pre-commit check with --no-verify.
Anyways the only error I couldn't fix was this section because it would parse the html instead of showing it in the codeblock:
https://github.com/thepracticaldev/dev.to/blob/7c369facf183fb236f47333e0fe913e565b12d2a/app/views/pages/_editor_guide_text.html.erb#L192-L194

Related Tickets & Documents

Closes #1760

Mobile & Desktop Screenshots/Recordings (if there are UI changes)

Added to documentation?

• help tab in the markdown editor
• docs.dev.to
• readme
• no documentation needed

[optional] What gif best describes this PR or how it makes you feel?

[Zhao-Andy]

LGTM!