Implement needed User routes for registration, activation and passwordreset | Follow up for #11

Regarding to #11 (closed)
Note: Add expiration to the activation key (frag.jetzt#153 (closed)) Userprofile:

• POST /user/registered (was /user/{loginprovider} in the old backend)
• POST /user/register
• POST /user/~{email}/activate
• POST /user/{id}/resetpassword