Various bugs/improvements

[aguillon]

While using the API directly to manage contracts (no front-end), I noticed

• we return a HTTP 500 when trying to create a user that already exists
• we should add a route to get the Tezos address of the API to allow deposits

to be continued

[aguillon]

• migrations are no bueno at the moment, we'll rewrite them from scratch so that they can be executed from an empty DB