Login

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Description:
Processes the Login Form to log a user in and create a session which holds the user's username and usertype.

Accessible by:
Users who are not logged in

Accessed from:
Login Form

Input parameters:
Username
Password

Output
If login is successful, either redirects to Main Menu or Booking Form (if the user was forced to log in because they tried to book a flight)
If login is unsuccessful, displays the Login Form with an error message.

Pseudo Code:

get/create session
if (there is no 'username' parameter in the session) { //i.e. user is not logged in
   get form parameters
   if (form parameters are in the correct format) { //guard against manually generated params
      if (the submitted username/password combination exists in the database) {
         add the username and user type to the session
         if (there is a 'Booking' parameter in the session) { // user is trying to book a flight
            display success message and redirect to Booking Form
         }
         else {
            display success message and redirect to Main Menu
         }
      }
      else {
         display Login Form with an error message
      }
   }
   else {
      display Login Form with an error message
   }
}
else {
   display an error message
}