[REF] Fix Accessibility issues with using single email / email to 50 …

[seamuslee001]

…contacts action

Overview

This fixes some missing labels causing Accessibility issues. Note that this requires colemanw/select2#6

Before

Missing form labels

After

Wave shows no errors

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[vingle]

test this please

[vingle]

This looks error free other than the schedule-follow-up label, which i think is waiting on the Select2 fix.

[colemanw]

@seamuslee001 I agree this looks like it will work, but I'm confused about one thing: why do you sometimes use class="sr-only" but other times you use an inline style style="visibility: hidden; display: none;". Seems like they'd both do the same thing & so probably better to always use the class?