Improve registration mail

Better wording and use shared signature.

weblate/html/mail/signature.txt

-{% load url from future %}{% load i18n %}
--- 
-    {% trans "Sent by Weblate translation system" %}
-    http://weblate.org/
+{% load url from future %}{% load translations %}{% load i18n %}--
+    {% blocktrans %}Sent by Weblate translation system{% endblocktrans %}
+    http://{{ site.domain }}{% url 'home' %}
+    {% trans "More information about Weblate can be found at http://weblate.org/" %}

weblate/html/registration/activation_email.txt

-{% load url from future %}{% load i18n %}{% load translations %}{% filter wordwrap:72 %}{% blocktrans with site|site_title as site_title %}Hi,
+{% load url from future %}{% load i18n %}{% load translations %}{% autoescape off %}{% filter wordwrap:72 %}{% trans "Hi," %}
 
-This is an automatic email to help you complete your registration
-with {{ site_title }}.
+{% blocktrans with site|site_title as site_title %}This is an automatic email to help you complete your registration with {{ site_title }}.{% endblocktrans %}
 
-Please open the following link in your web browser. If the link
-is split over several lines, you may need to copy it in the
-address bar.{% endblocktrans %}
+{% blocktrans %}Please open the following link in your web browser. If the link is split over several lines, you may need to copy it in the address bar.{% endblocktrans %}
 
 http://{{ site.domain }}{% url 'registration.views.activate' activation_key=activation_key %}
 
-{% blocktrans with site|site_title as site_title %}
-If there is a problem with your registration, please reply to
-this email.
+{% blocktrans %}If there is a problem with your registration, please contact us:{% endblocktrans %}
 
-Best regards,
-{{ site_title }}
-{% endblocktrans %}{% endfilter%}
+http://{{ site.domain }}{% url 'contact' %}?subject=Registration
+
+{% endfilter%}{% endautoescape %}{% include "mail/signature.txt" %}