Migration Rewards Vault

[jalextowle]

This PR adds the MigrationRewardsVault. This vault locks the depositor's ELFI for 3 months and then unlocks a 5% APR bonus over the following two months. If users migrate after the cliff, their bonus period will start later, and they will receive a smaller bonus than if they had locked their ELFI earlier. Once the users claim their HD tokens, they stop receiving a bonus.

[jrhea]

This is great work! Ideally, it would be good to have the following tests:

• migrate pre-cliff, (attempt) claim pre-cliff
• migrate pre-cliff, claim post-cliff, pre-expiration
• migrate pre-cliff, claim post-expiration
• migrate post-cliff, pre-expiration, claim pre-expiration
• migrate post-cliff, pre-expiration, claim post-expiration
• migrate post-expiration, claim post-expiration

to verify that all the logic works as expected. Wdyt?

[jrhea]

approved! just a couple of additional nits