Add option to copy existing template when adding

Sometimes when setting up a service you might have a few very similar
templates, in which only a small amount of content. Or you might even
have a few of services, which are used by different teams but have
similar templates.

Copy and pasting, especially from one service to another, is a pain.
This commit makes it easier by allowing users to copy an existing
template when choosing to add a new one, instead of starting from
scratch.

app/main/forms.py

@@ -837,14 +837,15 @@ class ChooseTemplateType(StripWhitespaceForm):
         ]
     )
 
-    def __init__(self, include_letters=False, *args, **kwargs):
+    def __init__(self, include_letters=False, include_copy=False, *args, **kwargs):
 
         super().__init__(*args, **kwargs)
 
         self.template_type.choices = filter(None, [
             ('email', 'Email'),
             ('sms', 'Text message'),
-            ('letter', 'Letter') if include_letters else None
+            ('letter', 'Letter') if include_letters else None,
+            ('copy-existing', 'Copy of an existing template') if include_copy else None,
         ])