Auth update

[Zilola-Nazarova]

This issue was created in the middle of the work when we realized that we might level up our authentication process.

This branch is completely based on the tasks brainstormed during the morning meeting:

JWT:

• [x] write a function that encodes the user's ID to Token
• [x] write a function that decodes the user's ID from Token

Sessions Controller:

• [x] add the step to #create action that generates Token (using JWT) and returns it in JSON

Application Controller:

• [x] write a function that:

  • extracts Token from request
  • decodes the user's ID from the Token
  • finds the User by decoded ID
  • sets the @current_user