Claim reward button is never disabled

[matevz]

Regression caused by e4a213c

Disabled version of the "Reclaim reward" button is never shown because we don't test isClaimingReward here:

demo-quiz/frontend/src/views/QuizView.vue

Lines 447 to 457 in 974edd5

	<AppButton
	class="mb-20 no-capitalize"
	type="submit"
	variant="primary"
	:disabled="isAddingReward"
	>
	<span class="normal-case" v-if="isAddingReward"
	>Generating transaction and sending reward…</span
	>
	<span class="normal-case" v-else>Claim your reward</span>
	</AppButton>

Maybe we can join the isAddingReward and isClaimingReward variables into a single one?