Allow for case-insensitive match of GitHub usernames stored in users.txt

GitHub is case-preserving on usernames, but '@rtyley' and '@RTYLEY'
are still the same account (GitHub don't redirect
https://github.com/RTYLEY to https://github.com/rtyley, but they
probably should- the pages contain the same user info).

After seeing a user incorrectly booted off an org because their username
was entered with incorrect capitalisation in the users.txt file, adding
case-insensitive username matching.

https://github.com/guardian/people/issues/337
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