Use a default path in the editor

When opening a think with no path specified, try some of the most common
paths and show the first one that exists.

thinks/views.py

@@ -82,6 +82,12 @@ class RemixThinkView(ThinkMixin, generic.UpdateView):
 class ThinkView(ThinkMixin, generic.DetailView):
     template_name = "thinks/think.html"
 
+    default_paths = [
+        'src/App.elm',
+        'script.js',
+        'index.html',
+    ]
+
     def get_context_data(self, *args, **kwargs):
         context = super().get_context_data(*args, **kwargs)
 
@@ -93,6 +99,12 @@ class ThinkView(ThinkMixin, generic.DetailView):
 
         path = think.file_path(strpath)
 
+        if path is None:
+            for f in self.default_paths:
+                if (root / f).exists():
+                    path = root / f
+                    break
+
         relpath = path.relative_to(root) if path is not None else None
 
         if path is None: