Add endpoint to get allowance history (#37)

Closes #12

Reviewed-on: #37

backend/dto.go

@@ -1,5 +1,7 @@
 package main
 
+import "time"
+
 type User struct {
 	ID   int    `json:"id"`
 	Name string `json:"name"`
@@ -12,8 +14,8 @@ type UserWithAllowance struct {
 }
 
 type Allowance struct {
-	Allowance int    `json:"allowance"`
-	Goals     []Goal `json:"goals"`
+	Allowance int       `json:"allowance"`
+	Timestamp time.Time `json:"timestamp"`
 }
 
 type PostAllowance struct {