Registering users via REST

As a JTalks Component developer I'd like to be able to register a user in Poulpe via REST API so that this user is shared across all the JTalks Components
Acceptance Criteria:

• URI: /rest/user
• POST method should be used
• Positive outcome: HTTP 200 Ok
• Negative outcomes: HTTP 400 Bad Request. Error messages depending on the particular problem should be taken from JTalks Common:
  • user.email.invalid_format - take validation regex from JCommune
  • user.email.length_constraint_violation - 50 symbols
  • user.email.already_exists
  • user.password.length_constraint_violation - note, that JTalks Component is going to send an MD5 hash instead of open password which should be stored as is. Even though it's always 32 symbols long, we need to support 50 symbols (later expected to make 255). This is important because in the future we'll change our hash algorithm.
  • user.username.length_constraint_violation - right now it's 1..25 symbols, let's keep it like that atm.
  • user.username.already_exists

Input message:

<user>
  <username>...</username>
  <passwordHash>...</passwordHash>
  <email>...</email>
</user>