Remove hard-coded bank holidays

Looks like someone in the Ministry of Justice has made a nice little
Python package of them.

I’ve configured it to use the cached holidays that come with the
package, rather than going to the GOV.UK website every time we start the
app. This should be more reliable, and means we’ll only get updates when
we do a version bump (PyUp should keep an eye on this for us).

requirements.txt

@@ -4,6 +4,7 @@
 # with package version changes made in requirements-app.txt
 
 ago==0.0.93
+govuk-bank-holidays==0.6
 humanize==2.2.0
 Flask==1.1.1
 Flask-WTF==0.14.3