Add opengraph tags to some pages

Add template support for opengraph tags (which will enable things like
icons and preview on posts to twitter and facebook and other places) to
both base pages and documentation pages. Use this to actually add the
tags to docs pages (only v10+ will get text previews, but all should get
the titles and image) and news (only news from the project will get an
image, all get both title and text)

pgweb/docs/views.py

@@ -2,6 +2,7 @@ from django.shortcuts import render, get_object_or_404
 from django.http import HttpResponseRedirect, HttpResponsePermanentRedirect
 from django.http import HttpResponse, Http404
 from pgweb.util.decorators import login_required, allow_frames, content_sources
+from django.template.defaultfilters import strip_tags
 from django.db.models import Q
 from django.conf import settings
 
@@ -111,6 +112,14 @@ def docpage(request, version, filename):
         params=[fullname, fullname, fullname],
     ).order_by('-version__supported', 'version').only('version', 'file')
 
+    # If possible (e.g. if we match), remove the header part of the docs so that we can generate a plain text
+    # preview. For older versions where this doesn't match, we just leave it empty.
+    m = re.match(r'^<div [^>]*class="navheader"[^>]*>.*?</div>(.*)$', page.content, re.S)
+    if m:
+        contentpreview = strip_tags(m.group(1))
+    else:
+        contentpreview = ''
+
     return render(request, 'docs/docspage.html', {
         'page': page,
         'supported_versions': [v for v in versions if v.version.supported],
@@ -119,6 +128,13 @@ def docpage(request, version, filename):
         'title': page.title,
         'doc_index_filename': indexname,
         'loaddate': loaddate,
+        'og': {
+            'url': '/docs/{}/{}'.format(page.display_version(), page.file),
+            'time': page.version.docsloaded,
+            'title': page.title.strip(),
+            'description': contentpreview,
+            'sitename': 'PostgreSQL Documentation',
+        }
     })