possibility to rename user

added a function in the detail view of each user to rename him.
* I first tried to use .update() or .replace() in the rethinkdb but since the "name" is the primary key and those cannot be changed in rethinkdb, I needed to .insert() a new user and .delete() the old one.
* That probably messes up the transactions table since it references users which are no longer present, so there might be some re-adjustments to the transaction table necessary…