UI Switch - More reliable onChange handler & accompanying E2E Test

[joepavitt]

Description

• We previously had an @click event handler which watched the click event, then invertted the current value and emitted it. It would appear, for some reason that the @click handler sometimes ran after the underlying Vue tick had computed, and sometimes before.
• I've changed this to a watch on the attached value to the switch, which guarantees we see an accurate value in the event handler, even on reload.

Related Issue(s)

Closes #457

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass