Expose build commit/branch to templates via SITE_COMMIT/SITE_BRANCH

pelicanconf.py reads them from env vars (blank on a plain local
build); the CI workflow's Build step now sets SITE_COMMIT (short SHA)
and SITE_BRANCH before invoking pelican. Bumps the theme submodule to
pick up the matching footer change (427fd9d).

pelicanconf.py

@@ -2,6 +2,8 @@
 # -*- coding: utf-8 -*- #
 from __future__ import unicode_literals
 
+import os
+
 AUTHOR = 'haemka'
 SITENAME = '[hɐəmkɑ]'
 SITEURL = 'http://haemka-de.wd.haemka.lan'
@@ -50,6 +52,11 @@ SOCIAL = (
     ('Matrix', 'matrix:u/haemka:matrix.org', 'matrix'),
 )
 
+# Build info shown in the footer, set by the CI workflow (blank on a
+# plain local build).
+SITE_COMMIT = os.environ.get('SITE_COMMIT', '')
+SITE_BRANCH = os.environ.get('SITE_BRANCH', '')
+
 DEFAULT_PAGINATION = False
 
 RELATIVE_URLS = True