Add encrypted 'Invite Request' message

[adilwali]

As the network grows, we cannot simply rely on an Alias to identify a user in an unlock request to another user.

Further, as public figures or people with large internet followings share Merit, they are less likely to know each user individually. For them to be stewards of the community, they need to evaluate users based on some criteria other than the alias name itself. An encrypted message that is limited in size should allow for this. Users can explain in this message either who they are are why they are a good potential member for the Merit community.

It serves two purposes:

1. To identify the user in the case that they have no alias or that their alias is not recognizable to the prospective inviter.
2. To empower inviters who do not know the requester directly to evaluate them.

Once implemented, the Wallet UIs should ensure that users either have an alias or provide an invite-request message.