Simplify jinja2 base templates, update theme options

- New realfavicongenerator.net packages (https://realfavicongenerator.net/blog/new-favicon-package-less-is-more/)
- Support all possible Pelican feed links
- Add recommended HEAD elements (https://github.com/joshbuchea/HEAD)
- Use minified css assets
- Link to boostrap, clean-blog and font-awesome stylesheets
- Add theme options, better options names

Template consistency, update structure on base.html

Fix missing article summary on index.html

Show the article.summary for teasers

alchemy/templates/authors.html

@@ -1,14 +1,17 @@
 {% extends "base.html" %}
 
 {% block title %}
-  {{ SITENAME }} — Authors
+Authors {{ super() }}
+{% endblock %}
+
+{% block page_header %}
+Authors
 {% endblock %}
 
 {% block content %}
-  <h1>Authors on {{ SITENAME }}</h1>
-  <ul class="list-unstyled">
-    {% for author, articles in authors|sort %}
-      <li><a href="{{ SITEURL }}/{{ author.url }}">{{ author }}</a> ({{ articles|count }})</li>
-    {% endfor %}
-  </ul>
+<ul class="list-unstyled">
+  {% for author, articles in authors|sort %}
+  <li><a href="{{ SITEURL }}/{{ author.url }}">{{ author }}</a> ({{ articles|count }})</li>
+  {% endfor %}
+</ul>
 {% endblock %}