(Feature) Allow displaying `RewardByBlock` item in drop-down list on the page `Modify Proxy Contract Ballot` for all networks

[varasev]

• (Mandatory) Description

  These changes are for displaying RewardByBlock item in the drop-down list on the page Modify Proxy Contract Ballot for Core, Sokol, or xDai network. Related to https://github.com/poanetwork/poa-network-consensus-contracts/pull/205, https://github.com/poanetwork/poa-dapps-voting/pull/194, https://github.com/poanetwork/poa-dapps-voting/pull/196.

  Should only be merged after the Proxy Ballot #0 in Core network is finalized.

• (Mandatory) What is it: (Fix), (Feature), or (Refactor) (Feature)

[varasev]

Validators finalized the ballot. @vbaranov please approve this PR.