Change max_proposal_period parameter to 260

[Gavin]

agree! there’s a social commitment to increasing the minimum to 7.25 days in the Phase 5 proposal, which would be just about 5 days voting min for validators

Mainnet parameter discussions: Knowable's proposal

Thanks @brentstone! I think I can say that we (Knowable) are ready to proceed with these parameters. Noting that: max_proposal_code_size (cc @Rigorous) turns out to be what we intended, being denominated in bytes in the code (but good catch, Rigorous! the docstring had mistakenly noted kb). Governance voting period would only be 3 day minimum voting at launch, which we think that should later increase to 7.25 days at the same time that transfers are enabled. Governance grace period would be 0…