Add British Museum to whitelist

> BM is an executive non-departmental public body, sponsored by the
> Department for Digital, Culture, Media & Sport.
>
> British Museum has a separate website (http://www.britishmuseum.org)

– https://www.gov.uk/government/organisations/british-museum

app/config.py

@@ -97,6 +97,7 @@ class Config(object):
         r"gov\.wales",
         r"biglotteryfund\.org\.uk",
         r"marinemanagement\.org\.uk",
+        r"britishmuseum\.org",
     ]
 
     LOGO_UPLOAD_BUCKET_NAME = 'public-logos-local'