Add aria-label="Add expense" for #add-button

[limonte]

Fixes #91

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This change is 

[manolo]

needs also to be applied to the pro version

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[limonte]

Done: https://github.com/vaadin/expense-manager-demo-pro/pull/16

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[yuriy-fix]

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[yuriy-fix]

Reviewed 1 of 1 files at r1. Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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