Show form elements for templates and folders

This commit adds:
- checkboxes to let you select a template or folder
- radio buttons to let you select where to move those template(s) and/or
  folder(s) to

It only does the `get` part of this work; handling the `post` and
calling API will be done in a subsequent commit.

app/main/forms.py

@@ -22,6 +22,7 @@ from wtforms import (
     IntegerField,
     PasswordField,
     RadioField,
+    SelectMultipleField,
     StringField,
     TextAreaField,
     ValidationError,
@@ -30,6 +31,7 @@ from wtforms import (
 )
 from wtforms.fields.html5 import EmailField, SearchField, TelField
 from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
+from wtforms.widgets import CheckboxInput, ListWidget
 
 from app.main.validators import (
     Blacklist,
@@ -102,6 +104,11 @@ def get_next_days_until(until):
     ]
 
 
+class MultiCheckboxField(SelectMultipleField):
+    widget = ListWidget(prefix_label=False)
+    option_widget = CheckboxInput()
+
+
 def email_address(label='Email address', gov_user=True):
     validators = [
         Length(min=5, max=255),
@@ -1111,3 +1118,41 @@ class ReturnedLettersForm(StripWhitespaceForm):
 
 class TemplateFolderForm(StripWhitespaceForm):
     name = StringField('Folder name', validators=[DataRequired(message='Can’t be empty')])
+
+
+class TemplateAndFoldersSelectionForm(Form):
+
+    ALL_TEMPLATES_FOLDER = {
+        'name': 'All templates',
+        'id': None,
+    }
+
+    def __init__(
+        self,
+        service,
+        template_type,
+        current_folder_id,
+        *args,
+        **kwargs
+    ):
+
+        super().__init__(*args, **kwargs)
+
+        self.templates_and_folders.choices = self.ids_and_names(
+            service.get_template_folders_and_templates(template_type, current_folder_id)
+        )
+
+        self.move_to.choices = self.ids_and_names(
+            [self.ALL_TEMPLATES_FOLDER] + service.all_template_folders,
+            exclude=current_folder_id,
+        )
+
+    @staticmethod
+    def ids_and_names(items, exclude=None):
+        return [
+            (item['id'], item['name']) for item in items
+            if item['id'] != exclude
+        ]
+
+    templates_and_folders = MultiCheckboxField('Choose templates or folders')
+    move_to = RadioField('Choose a folder')