Use BASE_URL in password reset email if available - fixes #841

wagtail/wagtailadmin/templates/wagtailadmin/account/password_reset/email.txt

@@ -1,3 +1,3 @@
-{% load i18n %}
+{% load i18n wagtailadmin_tags %}{% base_url_setting as base_url %}
 {% trans "Please follow the link below to reset your password" %}
-{{ protocol }}://{{ domain }}{% url 'wagtailadmin_password_reset_confirm' uidb64=uid token=token %}
+{% if base_url %}{{ base_url }}{% else %}{{ protocol }}://{{ domain }}{% endif %}{% url 'wagtailadmin_password_reset_confirm' uidb64=uid token=token %}

wagtail/wagtailadmin/templatetags/wagtailadmin_tags.py

@@ -134,6 +134,11 @@ def usage_count_enabled():
     return getattr(settings, 'WAGTAIL_USAGE_COUNT_ENABLED', False)
 
 
+@register.assignment_tag
+def base_url_setting():
+    return getattr(settings, 'BASE_URL', None)
+
+
 class EscapeScriptNode(template.Node):
     TAG_NAME = 'escapescript'
     SCRIPT_RE = re.compile(r'<(-*)/script>')