fix(v2): less strict blog/docs uri frontmatter validation

[slorber]

Motivation

After the beta.1 release, it has been reported that the new frontmatter validation is a bit too strict, in particular for URIs in blog post image/author URL. This makes the validation less strict, and allows paths like /x/y.png as before

#5011
#5012

Note: Joi is really painful to work with, in particular for the uri() type which cannot accept an absolute path, and does not make it easy at all to have clean error messages when building custom validation logic.

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

[netlify]

✔️ [V2]

🔨 Explore the source changes: f553169

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60d19f80213eff00078408cd

😎 Browse the preview: https://deploy-preview-5032--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	54
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5032--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 569 kB

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	87.8 kB
website/build/assets/js/main.********.js	385 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	65.8 kB
website/build/docs/introduction/index.html	235 B
website/build/index.html	30.5 kB

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[dschaller]

Thanks for the fix! Do you know when this will be released?