feat: add microdatas

[Kocal]

Summary

This PR adds some microdatas on post and posts list. Microdatas are useful for search engines/web crawlers and can improve your position on search engines or the user experience.

Here are some screenshots on the Google's Structured data Testing tool:

Homepage (before):

Homepage (after):

Post (before):

Post (after):

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [ ] Build-related changes
• [ ] Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

No UI changes.

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• [x] Chrome 79.0.3945.130
• [ ] Firefox
• [ ] Safari
• [ ] Edge
• [ ] IE

If adding a new feature, the PR's description includes:

• [x] A convincing reason for adding this feature
• [x] Related documents have been updated
• [x] Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

Nope.

[billyyyyy3320]

Nice! I'll review it ASAP.

[Kocal]

Why not adding publisher property to author, too?

I wasn't really sure of how to use the microdata properly, but sure we can add it.

Is dateModified really required since we're currently only displaying published date?

You're right, it's not displayed but I thought it can be intersting to still have the info somewhere. Maybe we should display it?

[billyyyyy3320]

IMO, we can provide an option whether to display last modified date, but it'll be another PR or issue, so leave dateModified to it.

[Kocal]

I've added "publisher" micro-data:

[billyyyyy3320]

Cool, could you remove dateModified for now, and I'll merge it.

[Kocal]

Done