UI: Make permalink button actual link

This allows to use browser copy link feature or open in new tab.

Fixes #4191

weblate/templates/translate.html

@@ -68,7 +68,7 @@
 <div class="panel {% if unit.approved %}panel-success{% elif unit.translation.is_source %}panel-warning{% else %}panel-default{% endif %}">
 
   <div class="panel-heading">
-  <a class="btn btn-link btn-xs pull-right flip" data-clipboard-text="{{ site_url }}{{ unit.get_absolute_url }}" title="{% trans "Copy permalink" %}">{% icon "link.svg" %}</a>
+  <a class="btn btn-link btn-xs pull-right flip" href="{{ site_url }}{{ unit.get_absolute_url }}" data-clipboard-text="{{ site_url }}{{ unit.get_absolute_url }}" title="{% trans "Copy permalink" %}">{% icon "link.svg" %}</a>
   <h4 class="panel-title">
     {% if unit.translation.is_source %}
         {% trans "Source string" %}