Add button widget for forms

This widget will allow displaying HTML anchor buttons inside
the forms. It uses by default the navigation instance template
from the navigation app.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/common/templates/common/forms/widgets/button_widget.html

@@ -0,0 +1,3 @@
+{% with widget.value as link %}
+    {% include 'navigation/generic_link_instance.html' %}
+{% endwith %}

mayan/apps/common/widgets.py

@@ -9,6 +9,15 @@ from .icons import icon_fail as default_icon_fail
 from .icons import icon_ok as default_icon_ok
 
 
+class ButtonWidget(forms.widgets.Widget):
+    template_name = 'common/forms/widgets/button_widget.html'
+
+    def format_value(self, value):
+        if value == '' or value is None:
+            return None
+        return value
+
+
 class DisableableSelectWidget(forms.SelectMultiple):
     allow_multiple_selected = True