views: Fix imports of REQ/has_request_variables from the wrong place.

These were never in zerver/decorator.py, and so it makes sense to
import them zerver/lib/request.py, mostly for ease of finding things.

zerver/views/attachments.py

@@ -1,9 +1,9 @@
 from django.http import HttpRequest, HttpResponse
 
-from zerver.decorator import REQ
 from zerver.models import UserProfile
 from zerver.lib.validator import check_int
 from zerver.lib.response import json_success
+from zerver.lib.request import REQ
 from zerver.lib.attachments import user_attachments, remove_attachment, \
     access_attachment_by_id