Change gas cost of `init-proposal`

[brentstone]

The gas cost for tx_init_proposal should be changed. Important points:

• It costs too much (sometimes exceeding block limits) to put large proposals on-chain
• Recommend to make the gas cost a small flat number, independent of the size of the attached wasm data
  • There is already a protocol limit of the wasm size governed by max_proposal_code_size
  • DoS protection already provided by the amount of tokens a proposal submitter must escrow - min_proposal_fund
• Makes it significantly easier to submit gov proposals with wasm code and keep the desired max_block_gas