Added social_url helper (#572)

ref https://linear.app/ghost/issue/PLG-412/create-template-helpers

- Instead of duplicating facebook_url and twitter_url we're creating a new
  social_url helper that takes a social type and then outputs the right url
- It uses the context provided, the expected usage would be
{{#if bluesky}}
  <a class="gh-author-bluesky" href="{{social_url type="bluesky"}}" target="_blank" rel="noopener">{{> icons/bluesky}}</a>
{{/if}}

Author: ErisDS

lib/specs/canary.js

@@ -15,7 +15,7 @@ const authorHelperDocs = `Find more information about the <code>{{authors}}</cod
 const tierDesc = `Ghost now supports multiple tiers and subscriptions. All product and price related helpers have been reworked to account for this.`;
 
 // assign new or overwrite existing knownHelpers, templates, or rules here:
-let knownHelpers = ['total_members', 'total_paid_members', 'comment_count', 'comments', 'recommendations', 'readable_url', 'content_api_url', 'content_api_key'];
+let knownHelpers = ['total_members', 'total_paid_members', 'comment_count', 'comments', 'recommendations', 'readable_url', 'content_api_url', 'content_api_key', 'social_url'];
 let templates = [];
 let rules = {
     // New rules

test/fixtures/themes/080-helper-usage/v4/invalid/index.hbs

@@ -5,5 +5,7 @@
 
 {{tag.feature_image}}⁠⁠
 
+{{social_url type="facebook"}}
+
 {{> mypartial}}
 {{> author}}

test/fixtures/themes/080-helper-usage/v5/valid/index.hbs

@@ -3,4 +3,6 @@
 
 {{tag.feature_image}}⁠⁠
 
+{{social_url type="facebook"}}
+
 {{> mypartial}}