Allow depositing vesting tokens in the vault

[ethanfrey]

Builds on #89 but making the vault even closer to native staking.

Situation: I have 1000 vesting $OSMO, which I could stake normally on one validator. It is currently not staked at all.

Problem: I wish to use these tokens for mesh-security so I can also stake on Juno and Akash, however, I am unable to deposit these tokens in the vault.

There is no clear design how to change this as this would involve deep changes in the bank / auth module to change how vesting works. This issue serves to track any design work or ideas on how to make this possible.