Split email list out again

The list of email domains is a different list from the list of all
government domains. And because the list of all government domains is
really long now, it could be unnecessarily slow to search through when
(a lot of the time) all we care about is whether the email address ends
with `.gov.uk`.

app/email_domains.yml

@@ -0,0 +1,30 @@
+---
+- gov.uk
+- mod.uk
+- mil.uk
+- ddc-mod.org
+- slc.co.uk
+- gov.scot
+- parliament.uk
+- nhs.uk
+- nhs.net
+- police.uk
+- dclgdatamart.co.uk
+- ucds.email
+- naturalengland.org.uk
+- hmcts.net
+- scotent.co.uk
+- assembly.wales
+- cjsm.net
+- cqc.org.uk
+- bl.uk
+- stfc.ac.uk
+- wmfs.net
+- bbsrc.ac.uk
+- acas.org.uk
+- gov.wales
+- biglotteryfund.org.uk
+- marinemanagement.org.uk
+- britishmuseum.org
+- derrystrabane.com
+- highwaysengland.co.uk