Change CSV export to buffer instead of streaming

The idea behind a streaming CSV export was to reduce the amount of
memory used, by avoiding building the entire CSV file in memory before
sending it to the client. However, it didn't work out this way in
practice: the query objects that were created to represent each line
caused Postgres to generate a very large (~200MB on bookwyrm.social)
temp file, not to mention the memory being used by the Query object
likely being similar to, if not larger than that used by the finalized
CSV row.

While we should in the long term run our CSV exports as a Celery task,
this change should allow CSV exports to work on large servers without
causing disk-space problems.

Fixes: #2157

bookwyrm/tests/views/preferences/test_export.py

@@ -1,7 +1,7 @@
 """ test for app action functionality """
 from unittest.mock import patch
 
-from django.http import StreamingHttpResponse
+from django.http import HttpResponse
 from django.test import TestCase
 from django.test.client import RequestFactory
 
@@ -57,13 +57,12 @@ class ExportViews(TestCase):
         request = self.factory.post("")
         request.user = self.local_user
         export = views.Export.as_view()(request)
-        self.assertIsInstance(export, StreamingHttpResponse)
+        self.assertIsInstance(export, HttpResponse)
         self.assertEqual(export.status_code, 200)
-        result = list(export.streaming_content)
         # pylint: disable=line-too-long
         self.assertEqual(
-            result[0],
-            b"title,author_text,remote_id,openlibrary_key,inventaire_id,librarything_key,goodreads_key,bnf_id,viaf,wikidata,asin,aasin,isfdb,isbn_10,isbn_13,oclc_number,rating,review_name,review_cw,review_content\r\n",
+            export.content,
+            b"title,author_text,remote_id,openlibrary_key,inventaire_id,librarything_key,goodreads_key,bnf_id,viaf,wikidata,asin,aasin,isfdb,isbn_10,isbn_13,oclc_number,rating,review_name,review_cw,review_content\r\nTest Book,,"
+            + self.book.remote_id.encode("utf-8")
+            + b",,,,,beep,,,,,,123456789X,9781234567890,,,,,\r\n",
         )
-        expected = f"Test Book,,{self.book.remote_id},,,,,beep,,,,,,123456789X,9781234567890,,,,,\r\n"
-        self.assertEqual(result[1].decode("utf-8"), expected)