[perilbot] validate updates to the site showcase

[m-allanson]

Summary

Use Gatsby's Peril setup to automatically check any pull request that modifies the list of showcase sites.

It should verify that added sites contain the correct fields - url, categories etc.

Basic example

Example message notifiying about missing fields:

Gadzooks! It looks like you forgot to add a link to your new site. You should update your pull request to include a url field that links to your site.

Stretch goals:

• check categories and suggest related categories, highlight possible typos or alert to categories where your site will be the only site in the category?
• check that urls are valid

Motivation

Missing fields mean the added site may not get published to the showcase.

[jlengstorf]

Gadzooks!

😂

I love this idea. I'm going to set up a project proposal to track ideas like these, and we'll see if we can get it prioritized.

[m-allanson]

Additional things Peril could do:

• automatically add a date_added field
• automatically add a gatsby_version field

(ref: #6930)

[jlengstorf]

@pieh Is this the one you wanted to tackle?