add back button in edit pages (#96)

closes #89

Reviewed-on: #96

frontend/allowance-planner-v2/src/app/pages/edit-allowance/edit-allowance.page.ts

@@ -1,3 +1,4 @@
+import { Location } from '@angular/common';
 import { Component, OnInit } from '@angular/core';
 import { FormBuilder, FormGroup, Validators } from '@angular/forms';
 import { ActivatedRoute, Router } from '@angular/router';
@@ -31,7 +32,8 @@ export class EditAllowancePage implements OnInit {
     private route: ActivatedRoute,
     private formBuilder: FormBuilder,
     private allowanceService: AllowanceService,
-    private router: Router
+    private router: Router,
+    private location: Location
   ) {
     this.userId = this.route.snapshot.params['id'];
     this.goalId = this.route.snapshot.params['goalId'];
@@ -98,4 +100,8 @@ export class EditAllowancePage implements OnInit {
     this.allowanceService.deleteAllowance(this.goalId, this.userId);
     this.router.navigate(['/tabs/allowance', this.userId]);
   }
+
+  navigateBack() {
+    this.location.back();
+  }
 }