[Change] Update Download batch CSV URL and add in API docs openwisp#373

* URL generated with radius:serve_private_file was not consistent with the rest of the API URLs.
* Changed Download batch CSV API endpoint from radiusbatch/csv/<uuid>/filename.csv to radius/organization/<slug:slug>/batch/<uuid:pk>/csv/filename.csv and added the same in API documentation as well.

Closes openwisp#373

docs/source/user/api.rst

@@ -694,6 +694,25 @@ Param              Description
 phone_number       string
 ===============    ===============================
 
+
+Batch CSV Download
+-------------------
+.. code-block:: text
+
+    /api/v1/radius/organization/<organization-slug>/batch/<id>/csv/<filename>
+
+Responds only to **GET**.
+
+Parameters:
+
+===============    ===============================
+Param              Description
+===============    ===============================
+slug               string
+id                 string
+filename           string
+===============    ===============================
+
 Batch user creation
 -------------------
 

openwisp_radius/base/models.py

@@ -822,7 +822,13 @@ def __str__(self):
 
 
 def _get_csv_file_location(instance, filename):
-    return os.path.join(str(instance.organization.pk), filename)
+    return os.path.join(
+        str(instance.organization.slug),
+        'batch',
+        str(instance.organization.pk),
+        'csv',
+        filename,
+    )
 
 
 class AbstractRadiusBatch(OrgMixin, TimeStampedEditableModel):

openwisp_radius/settings.py

@@ -131,7 +131,7 @@ def get_settings_value(option, default):
     )
 
 # Path of urls that need to be refered in migrations files.
-CSV_URL_PATH = 'api/v1/radiusbatch/csv/'
+CSV_URL_PATH = 'api/v1/radius/organization/'
 CALLED_STATION_IDS = get_settings_value('CALLED_STATION_IDS', {})
 CONVERT_CALLED_STATION_ON_CREATE = get_settings_value(
     'CONVERT_CALLED_STATION_ON_CREATE', False