Revert "Rename SITESUBTITLE to SITE_SUBTITLE"

This reverts commit e479ce72db916e7e21bc6ae63677221c8e290840.

As [@calfzhou](https://github.com/calfzhou) [pointed
out](https://github.com/talha131/pelican-elegant/commit/e479ce72db916e7e21bc6ae63677221c8e290840#commitcomment-6284503)

> In pelican's document, the default theme uses SITESUBTITLE.

RELEASE_NOTES.md

@@ -17,7 +17,6 @@ Version 2.0 (under development)
 * Add Disqus comments to Pages
 * All customizable variables consolidated in a single `_defaults.html`, making
   it easier for you to customize or even *localize* the theme
-* Rename `SITESUBTITLE` to `SITE_SUBTITLE`
 
 Performance
 -----------

templates/_includes/_defaults.html

@@ -99,10 +99,10 @@ search results and social media #}
 {% endif %}
 
 {# It is displayed along with the SITENAME in the footer of every page #}
-{% if not SITE_SUBTITLE %}
-{% set SITE_SUBTITLE = '' %}
+{% if not SITESUBTITLE %}
+{% set SITESUBTITLE = '' %}
 {% else %}
-{% set SITE_SUBTITLE = SITE_SUBTITLE %}
+{% set SITESUBTITLE = SITESUBTITLE %}
 {% endif %}
 
 {# Set it to True to display favicon and speed dial icon. Also make sure that

templates/_includes/footer.html

@@ -1,9 +1,9 @@
 <footer>
 <div id="footer">
     <ul class="footer-content">
-        {% from '_includes/_defaults.html' import SITE_SUBTITLE with context %}
-        {% if SITE_SUBTITLE %}
-        <li class="elegant-subtitle"><span class="site-name">{{ SITENAME }}</span> - {{ SITE_SUBTITLE }}</li>
+        {% from '_includes/_defaults.html' import SITESUBTITLE with context %}
+        {% if SITESUBTITLE %}
+        <li class="elegant-subtitle"><span class="site-name">{{ SITENAME }}</span> - {{ SITESUBTITLE }}</li>
         {% endif %}
         {% from '_includes/_defaults.html' import SITE_LICENSE with context %}
         {% if SITE_LICENSE %}