Add required meta tags for social media

[renintw]

Fixes #18

The preview of some posts couldn't be correctly displayed on some social media, so in this PR, required meta tags are added.

Note: A must-have twitter tag twitter:card is not added because it has already been added by the plugin social image generator.

Screenshot (Tested on sandbox)

Added by this PR ⬇️

Added by social image generator ⬇️

How to test

1. Locally or on the sandbox, choose a post.
2. Check the dev console if newly added tags are there.
3. Front page doesn't have a description tag at the moment.

TODO

1. Does the front page need a description? What should it be? (ref)
2. After the deployment, check if the issue is totally fixed as it was found out earlier that Slack still couldn't display the preview of a post correctly (Discord has no such problem though) while in the meantime the preview of the post mentioned in the issue was shown without any problem on my side. (Any thoughts? Perhaps it's img size restriction, I'll look further into it in this direction)

[StevenDufresne]

Disregard if I'm misunderstanding, but can we not just use the excerpt?

[renintw]

Thanks for pointing this out!

Originally this part was referred to a snippet in wporg-developer. The reason it parsed desc and trimmed it down to <150 chars with regex from scratch is that developer.w.org doesn’t store data within the post_content in a way that get_the_excerpt would handle well.

get_the_excerpt() would work well for the is_singular() if branch, but other parts it wouldn’t, so in the snippet the original author might just figure it was better to process all of the excerpts for SEO headers the same way.

I've changed to use get_the_exceprt() here as at the moment only is_singular is used and I'd like to get this PR merged first to see if the original issue is fixed.
Whether it needs some specific descriptions on different pages (front page, archive page, etc.), I'll have another discussion afterward to figure out the requirements.

Thanks to @dd32 for the clarification!