Add British Library to email domain whitelist

I think that British Library can use Notify. Need to double check
though.

> BL is an executive non-departmental public body, sponsored by the
> Department for Digital, Culture, Media & Sport.
> https://www.bl.uk/
— https://www.gov.uk/government/organisations/british-library

app/config.py

@@ -85,6 +85,7 @@ class Config(object):
         r"assembly\.wales",
         r"cjsm\.net",
         r"cqc\.org\.uk",
+        r"bl\.uk",
     ]
 
     LOGO_UPLOAD_BUCKET_NAME = 'public-logos-local'