Make all sniff codes case insensitive in rulesets

[jrfnl]

Repost from squizlabs/PHP_CodeSniffer#1955 by @gsherwood:

Everywhere a sniff code or error message code is sourced from a ruleset, do case insensitive matching on the real sniff and error codes. This will allow for easier creation and maintenance of rulesets and will stop issues like squizlabs/PHP_CodeSniffer#1909 from occurring in the future.

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I'm tentatively keeping this in the 4.0 milestone, but as I prefer to release 4.0 sooner rather than later, this issue may well move to the 5.0 milestone.