Align vote data structure (voting-system, frontend, backend)

Currently the vote data structure is inconsistent:

voting-system: Yes / No / Abstain
smart contract: Yes / No / Invalid

At least on smart-contract level we will need to support Yes / No / Abstain / Invalid. Ideally we unify the data structure across all layers.
 
**To-do:**

-  Define and implement unified vote data structure
