Fix alerts label/radio button Firefox bug

In Firefox, when an `input` is inside a `label`,
and the `label` is for a `radio` button that's
not that `input`, and that `input` is focused,
the focus shifts from the `input` to the `radio`
button, not allowing you to type within that input.
This is a known Firefox "bug":
https://bugzilla.mozilla.org/show_bug.cgi?id=213519

fixes CNVS-15519

test plan:
-Log in as account admin
-In account settings enable "Alerts (Beta)"
-Click on the "Alerts" tab and start to set up an alert.
-Try to type a value for "Re-send alert every __ days until resolved"
-Numeric value can be typed and saved.

Change-Id: I1d71d803b55879eaa0c279e19be136e7c3614570
Reviewed-on: https://gerrit.instructure.com/42186
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>