Get session secret from env instead of hardcoding

config/config.go

@@ -7,12 +7,14 @@ import (
 // Config contains all needed configurations.
 type Config struct {
 	DatabaseConnection string
+	SessionSecret      string
 }
 
 // LoadConfig from path.
 func LoadConfig() (*Config, error) {
 	configuration := Config{
 		DatabaseConnection: os.Getenv("BUDGETEER_DB"),
+		SessionSecret:      os.Getenv("BUDGETEER_SESSION_SECRET"),
 	}
 
 	return &configuration, nil