Investigate and implement MsgClaimInitial

[lumtis]

MsgClaimInitial allows claiming an initial airdrop portion from a mission that doesn't require specific action other than just sending a MsgClaimInitial message.

This feature is optional for project importing claim they may not eventually want to have a initial claim.

We should investigate how to best manage the mission for initial claim

Example:

• a predefined MissionID is reserved for initial claim: 0
• params define the mission ID reserved for initial claim