Generate new random identity when an order is created or taken

[Catrya]

There is no need for a user registration screen, nor for the user to be asked if they want to use a previously generated private key, or if they want to keep one as such

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No login screen required

[chebizarro]

PR #31 introduces the Session object which is an ephemeral key pair generated for each new order and is associated with that order through the order id. This allows each order to use a completely random identity which exists only for the duration of the order. Depending on the outcomes of the current discussion around User Identity and Reputation, this will most likely need to be revised, but the current implementation should address the issue as it is described.

[Catrya]

completed via https://github.com/chebizarro/mobile/releases/tag/v0.0.1-alpha.1