Prefer linking to GitHub profiles in blog post bylines (#408)

- Link to GitHub profile if present, to Twitter profile as fallback, reversing the current order
- Better supports viewing profile info even if not logged-in
  - Changes to Twitter/X now require being logged-in to see profile info
  - GitHub displays profile info even when not logged info

Author: dempseyatgithub

_layouts/post.html

@@ -18,10 +18,10 @@ <h1>{{ page.title }}</h1>
         {% endif %}
 
         <span class="author">
-          {% if author.twitter %}
-            <a href="https://twitter.com/{{ author.twitter }}/" rel="nofollow" title="{{ author.name }} (@{{ author.twitter}}) on Twitter">{{ author.name }}</a>
-          {% elsif author.github %}
+          {% if author.github %}
             <a href="https://github.com/{{ author.github }}/" rel="nofollow" title="{{ author.name }} (@{{ author.github}}) on GitHub">{{ author.name }}</a>
+          {% elsif author.twitter %}
+            <a href="https://twitter.com/{{ author.twitter }}/" rel="nofollow" title="{{ author.name }} (@{{ author.twitter}}) on Twitter">{{ author.name }}</a>
           {% else %}
             {{ author.name }}
           {% endif %}