Display the template folders nested on the move template/folder form

Before, all the folders were displayed in a list which was ordered but
not nested. This changes the move form to nest the template folders.

app/main/forms.py

@@ -742,6 +742,26 @@ class RadioFieldWithNoneOption(FieldWithNoneOption, RadioField):
     pass
 
 
+class NestedRadioField(RadioFieldWithNoneOption):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+
+    def children(self):
+        child_map = {}
+        child_ids = [
+            folder['id'] for folder in self.all_template_folders
+            if folder['parent_id'] is None]
+
+        child_map[None] = [option for idx, option in enumerate(self) if option.data in child_ids or idx == 0]
+
+        for option in self:
+            child_ids = [
+                folder['id'] for folder in self.all_template_folders
+                if folder['parent_id'] == option.data]
+            child_map[option.data] = [option for option in self if option.data in child_ids]
+        return child_map
+
+
 class HiddenFieldWithNoneOption(FieldWithNoneOption, HiddenField):
     pass
 
@@ -1207,7 +1227,6 @@ class TemplateAndFoldersSelectionForm(Form):
         self,
         all_template_folders,
         template_list,
-        current_folder_id,
         allow_adding_letter_template,
         allow_adding_copy_of_template,
         *args,
@@ -1221,13 +1240,10 @@ class TemplateAndFoldersSelectionForm(Form):
         self.op = None
         self.is_move_op = self.is_add_folder_op = self.is_add_template_op = False
 
-        if current_folder_id is None:
-            current_folder_id = RadioFieldWithNoneOption.NONE_OPTION_VALUE
-
+        self.move_to.all_template_folders = all_template_folders
         self.move_to.choices = [
             (item['id'], item['name'])
             for item in ([self.ALL_TEMPLATES_FOLDER] + all_template_folders)
-            if item['id'] != current_folder_id
         ]
 
         self.add_template_by_template_type.choices = list(filter(None, [
@@ -1262,10 +1278,12 @@ class TemplateAndFoldersSelectionForm(Form):
     templates_and_folders = MultiCheckboxField('Choose templates or folders', validators=[
         required_for_ops('move-to-new-folder', 'move-to-existing-folder')
     ])
-    move_to = RadioFieldWithNoneOption('Choose a folder', validators=[
-        Optional(),
-        required_for_ops('move-to-new-folder', 'move-to-existing-folder')
-    ])
+    move_to = NestedRadioField(
+        'Choose a folder',
+        validators=[
+            Optional(),
+            required_for_ops('move-to-new-folder', 'move-to-existing-folder')
+        ])
     add_new_folder_name = StringField('Folder name', validators=[required_for_ops('add-new-folder')])
     move_to_new_folder_name = StringField('Folder name', validators=[required_for_ops('move-to-new-folder')])