[request] Allow students to join the request by paying a minimum amount

[cosmin-harangus]

Students should be able to support and join a request for knowledge by paying a specific amount to the Request contract.

Define a method named Join() that is payable and records the amount of coins sent by every student.
This method should be secure and add the new student address to the list with his balance if he is new or update him amount if he has already paid.

The student should also be able to retreat their participation by calling a Leave() method that sends back his coins from the contract and removes him from the list of enrolled students and votes if the a teacher has not been selected yet.