fix(slide-toggle): label not being read out by screen reader on IE

[crisbeto]

Fixes the label of a slide toggle not being read out by NVDA on IE, because the label element isn't pointing to the input.

[devversion]

Technically the input wrapped inside of a <label> should be automatically associated with the label. Nevertheless, being explicit doesn't hurt. LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: tests_saucelabs" is failing
     status "ci/circleci: bazel_build_test" is pending
     status "ci/circleci: tests_local_browsers" is pending
     status "ci/circleci: tests_browserstack" is pending

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[crisbeto]

Yeah wrapping it should be enough, but it doesn't look like that's the case in IE. We've been doing the same in checkbox and radio already.

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