Add function to check if a string contains...

...one element in an array of strings.

Used to check if HTML response bodies contain one of the strings we
specify in an array.
Check for variations on 'Guru Meditation' errors

Our current CDN vendor, Fastly, currently use a custom error page in
which there is a typo. We'd normally expect a 'Guru Meditation' error,
but the typo results in a 'Guru Mediation' error.

Work around this by comparing the HTML response body with an array of
expected strings.
Merge pull request #78 from alphagov/76126570-guru-error-typo

[#76126570] Check for variations on 'Guru Meditation' errors