Update og-image to GitHub Classroom

[cyhsutw]

Fixes #565.

I've changed the title text of the og-image to GitHub Classroom.

Here's how it looks like now:

[tarebyte]

@cyhsutw thanks for the contribution!

I think the text color is off a bit (needs to be #9FD1AF) and the also make sure that we are using the latest octicons.

[johndbritton]

@tarebyte The wordmark was pure white in the previous version as well I think. This is slightly different than what we had, but I think it works.

We'll likely revisit the share image when we update the homepage.

[tarebyte]

@johndbritton 🆒 that's fine.

[cyhsutw]

Update:

I triggered the crawling process manually after this PR was merged.
However, it does not update the og:image immediately.
After two days, Facebook finally refreshed the cache, and the new image is now live.

According to this StackOverflow question, I think maybe we can change the name of the image a little bit if we need to update the og:image so the crawler will replace the cache immediately.