Meta tags: Change opengraph logo and add possiblity to use custom one

The old logo showing on social medias is too large and end up being trimmed. Replaced it with the simple Monero Logo.
If a logo is specified using 'logo:' in the front matter, jekyll will show this one instead. This also allow us to easily
make specific meta-logos for each blog posts. I will add the possibility to add the logo in the body of the post in a
subsequent PR.

README.md

@@ -190,6 +190,8 @@ author: YOUR NAME OR HANDLE HERE
 ---
 ```
 
+If you want to add a personalized picture to show as logo on social networks, add `icon: /blog/assets/$FOLDER/$IMAGE` to the front matter. Where `$FOLDER` is the name of the folder you created to contain the image related to your blog post and `$IMAGE` is the name of the image.
+
 ### 3. Write
 After the front matter is finished you are free to write the remainder of your blog post in markdown.
 

_includes/head.html

@@ -28,7 +28,8 @@
     {% elsif page.summary %}{{ page.summary }}
     {% else %}{% t global.titlemeta %}
     {% endif %}">
-    <meta property="og:image" content="{% if page.icon %}http://static.getmonero.org/images/opengraph/{{ page.icon }}.png{% else %}http://static.getmonero.org/images/opengraph/logo.png{% endif %}">
+    <!-- If the page specifies a dedicated logo we use that one, otherwise we use the classic Monero logo -->
+    <meta property="og:image" content="{% if page.icon %}https://getmonero.org{{ page.icon }}{% else %}https://getmonero.org/press-kit/symbols/monero-symbol-480.png{% endif %}">
     <meta property="og:site_name" content="{% t global.sitename %}">
     <meta property="og:url" content="https://getmonero.org{{ page.url }}">
     <meta property="og:type" content="website">

_posts/2020-05-23-monero-0.16-released.md

@@ -4,6 +4,7 @@ title: "Monero 0.16.0.0 'Nitrogen Nebula' released"
 summary: Major release with Dandelion++ integration, significant speedups and more
 tags: [releases]
 author: ErCiccione, selsta
+icon: /img/academic-growth.png
 ---
 
 # Overview