Add RewardsManager contract to manage DAO incentives for stakers

[oliviera9]

1. The new contract will receive tokens from whitelisted addresses, e.g. DAO voting contract.
2. A permission-less function can be called by anyone to register for rewards for a specific epoch.
3. One will be eligible for rewards if he has voted at least once in the specified epoch.
4. Once a reward is registered for a user, the contract will mint daoPNT tokens for that user so they will account in his balance.
5. Finally, the user will be able to withdraw his tokens after one year.

[gitmp01]

No major issues, few open question marks though.
Also, I didn't see integrations tests with the actual gnosis DandelioVotingContract, did you forget to push maybe?

[gitmp01]

LGTM