Remove VestingPeriod to provide the ending block relay number at initialization

[girazoki]

This PR does a couple of things:

• Divides initialize_reward_vec in initialize_reward_vec and complete_initialization. Both are root dispatchables, the first inserts contributors while the latter performs the necessary checks and closes initialization.
• Removes the VestingPeriod type. Instead, the vesting period is calculated against a lease_ending_block parameter provided in the complete_initialization extrinsic.

[JoshOrndorff]

I think the pallet is better with this PR than without it. So in that sense I approve. I also think we could make the initialization workflow even nicer.

What if we have two different extrinsics that are both useful for setting up the initial state, submit_initial_contribution_batch, and complete_initialization. The first one can be called many times and is just given a Vec of contributions (probably bounded to some max length) No more need for manually tracked indices etc. Then when all of the contributions have been submitted, a single call is made to complete_initialization. That call contains any only-needs-specified-once information such as the vesting period (or vestin end block, I have no preference).

Once copmlete_initialization is called, initial payouts are made and claims can start to be made. After this point any calls to either of the initialization extrinsics are invalid.

[girazoki]

Sounds good to me. You would include then the ending block number in the complete_initialization extrinsic?

[girazoki]

The PR has been adapted to this last idea. Let me know what you think!

[JoshOrndorff]

we would need a storage migration to fix the Moonriver deployment (right?)

This was already answered in chat, but I'll repeat it here. We won't need to migrate anything on Monriver because the crowdloan rewards haven't been initialized yet.