Add Site reference to Output, clean up template url handling

The following changes have been made all with template cleanup in mind:
- Make Output.url relative to the origin root
- Keep weakref to Site in all Output objects.
- Add several helpers to get urls from Site: home_url(), tag_url(tag),
  section_url(section) and tags_url().
- Add helpers to Index to get pagination urls: pagination_url(n)
- Clean up templates by making use of these new facilities

omicron/ssg/templates/plain/index.html

@@ -1,25 +1,22 @@
 {% extends "base.html" %}
-{% set base_path = ("/" + label) if kind == "section" else (("/tags/" + label) if kind == "tag" else "") %}
-{% set heading = label or "" %}
 
-{% block title %}{% if heading %}{{ heading }} — {% endif %}{{ site.name }}{% if page_num > 1 %} — Page {{ page_num }}{% endif %}{% endblock %}
+{% block title %}{% if page.label %}{{ page.label }} — {% endif %}{{ site.name }}{% if page.page_num > 1 %} — Page {{ page.page_num }}{% endif %}{% endblock %}
 
 {% block head %}
-<link rel="canonical" href="{{ site.origin }}{{ site.base_dir }}{% if page_num == 1 %}{{ base_path or "/" }}{% else %}{{ base_path }}/page-{{ page_num }}.html{% endif %}">
-{% if page_num > 1 %}<meta name="robots" content="noindex, follow">{% endif %}
+{% if page.page_num > 1 %}<meta name="robots" content="noindex, follow">{% endif %}
 {% endblock %}
 
 {% block breadcrumb %}
-{% if kind == "section" %}
+{% if page.kind == "section" %}
 <nav aria-label="breadcrumb">
-    <a href="{{ site.base_dir or "/" }}">{{ site.name }}</a> ›
-    {{ label }}
+    <a href="{{ site.home_url() }}">{{ site.name }}</a> ›
+    {{ page.label }}
 </nav>
-{% elif kind == "tag" %}
+{% elif page.kind == "tag" %}
 <nav aria-label="breadcrumb">
-    <a href="{{ site.base_dir or "/" }}">{{ site.name }}</a> ›
-    <a href="{{ site.base_dir }}/tags">Tags</a> ›
-    {{ label }}
+    <a href="{{ site.home_url() }}">{{ site.name }}</a> ›
+    <a href="{{ site.tags_url() }}">Tags</a> ›
+    {{ page.label }}
 </nav>
 {% else %}
 {{ super() }}
@@ -27,28 +24,28 @@
 {% endblock %}
 
 {% block main %}
-{% if heading %}<h1>{{ heading }}</h1>{% endif %}
-{% for item in items %}
+{% if page.label %}<h1>{{ page.label }}</h1>{% endif %}
+{% for item in page.items %}
 <article>
     <header>
-        <h2><a href="{{ site.base_dir }}{{ item.url }}">{{ item.title }}</a></h2>
+        <h2><a href="{{ item.url }}">{{ item.title }}</a></h2>
         <time datetime="{{ item.created }}">{{ item.created }}</time>
     </header>
     {% if item.summary %}<p>{{ item.summary }}</p>{% endif %}
 </article>
 {% endfor %}
-{% if total_pages > 1 %}
+{% if page.total_pages > 1 %}
 <nav>
-    {% if page_num > 1 %}
-    <a href="{{ site.base_dir }}{% if page_num - 1 == 1 %}{{ base_path or "/" }}{% else %}{{ base_path }}/page-{{ page_num - 1 }}.html{% endif %}">← Newer</a>
+    {% if page.page_num > 1 %}
+    <a href="{{ page.pagination_url(page.page_num - 1) }}">← Newer</a>
     {% endif %}
-    {% for p in range(1, total_pages + 1) %}
-    {% if p == page_num %}<strong>{{ p }}</strong>
-    {% else %}<a href="{{ site.base_dir }}{% if p == 1 %}{{ base_path or "/" }}{% else %}{{ base_path }}/page-{{ p }}.html{% endif %}">{{ p }}</a>
+    {% for p in range(1, page.total_pages + 1) %}
+    {% if p == page.page_num %}<strong>{{ p }}</strong>
+    {% else %}<a href="{{ page.pagination_url(p) }}">{{ p }}</a>
     {% endif %}
     {% endfor %}
-    {% if page_num < total_pages %}
-    <a href="{{ site.base_dir }}{{ base_path }}/page-{{ page_num + 1 }}.html">Older →</a>
+    {% if page.page_num < page.total_pages %}
+    <a href="{{ page.pagination_url(page.page_num + 1) }}">Older →</a>
     {% endif %}
 </nav>
 {% endif %}