Governance: Enable perpetual staking

JonnyPickles#2315 in Discord offered an interesting comment:

When user-selected staking period ends SOV becomes unstaked. I think the community and the protocol would both prefer SOV to stay staked and earning fees until the user unstakes... We want more people to be staked in more situations. Not less.

What if we changed the staking mechanism to something like this: users can "stake" their SOV for an indefinite period of time. Their voting power for their staked SOV would be equal to 1 voting power per 1 SOV staked, and they can unstake at any time with no penalty. However, the user can multiply their voting power if they lock their staked SOV for a given duration. Locking would multiply their voting power using the current quadratic formula we use to calculate voting power based on staking duration. Once their lock duration expires, their SOV is unlocked but remains staked, so they can continue earning staking rewards just at the lower 1:1 voting power level.

No files, tests, or entry points are named. Start by locating the staking mechanism and the current quadratic voting-power calculation, then clarify the intended contract behavior and acceptance tests. Done should define perpetual staking, expiry behavior, rewards, and unstaking semantics.