Add image number monitoring per user

[jreyesr]

Related to https://github.com/IS-AgroSmart/AgroSmart-Web/issues/114, there is a hard limit on the number of images that a user can submit per month. Otherwise, a user could submit unlimited tasks to NodeODM Lightning and (since we pay NodeODM Lightning per image) cost us more money than his monthly payment.

Backend

• [x] Add a remaining_images field on the User model, of type PositiveIntegerField, with a default value of 0.
• [x] Optional idea: also add a image_quota field, same type, that defaults to 3000.
• [x] Expose the remaining_images field on UserSerializer.
• [x] On the upload_images view, after L256, check if len(request.FILES) <= user.remaining_images. If not, raise an error HTTP 402, and return HttpResponse(f"Subida fallida. Tiene un límite de ${user.remaining_images} imágenes.", status=402). https://github.com/IS-AgroSmart/AgroSmart-Web/blob/c226455c9dcc93948d737b1349666fae23449540/core/views.py#L249-L259
• [x] Edit the previous HTTP 402 branch (on L256) and make it HttpResponse("Subida fallida. Su almacenamiento está lleno.", status=402)
• [x] If previous check passed, substract that number of images from the user (user.remaining_images -= len(request.FILES) and user.save()).
• [x] On the webhook, give the images back if flight status is CANCELED or ERROR (probably use data["imagesCount"]) from the NodeODM API) https://github.com/IS-AgroSmart/AgroSmart-Web/blob/7ee7507697b45996b04c43a55b5f7838b03e0b83/core/views.py#L277-L284
• [ ] [Requires Stripe integration] When the Stripe servers send a invoice.paid webhook, somehow parse the user from the response and reset his remaining_images to his image_quota (or 3000 if the image_quota is not implemented) (see https://briancaffey.github.io/2021/01/02/using-the-stripe-api-for-recurring-monthly-saas-subscription-payments-in-django-and-vue-application/#iii-automatic-subscription-renewal)
• [x] Create a Django Admin action to manually reset a user's image quota. The action should reset user.remaining_images to user.image_quota (or 3000 if the image_quota is not implemented). Also call user.save()!
• [x] Add tests:
  • [x] /upload-images returns 402 if user attempts to upload too many images (use users[1], check body of response)
  • [x] Images get substracted after successful upload
  • [x] Images get returned to the user after a failed flight
  • [x] Images DON'T get returned after a successful flight.

Frontend

On the frontend, we should add some client-side validation on the number of files selected, since there's no native way to do it :(

• [x] Add a new computed property, tooManyFiles() { return this.files.length > Math.min(this.$effectiveUser.remaining_images, 1000); }. https://github.com/IS-AgroSmart/AgroSmart-Web/blob/c226455c9dcc93948d737b1349666fae23449540/frontend/src/components/UploadImages.vue#L32-L37
• [x] Change the :state="enoughFiles" bit to be :state="enoughFiles && !tooManyFiles". Also change the error messages on the line below to include the case when tooManyFiles == true. https://github.com/IS-AgroSmart/AgroSmart-Web/blob/c226455c9dcc93948d737b1349666fae23449540/frontend/src/components/UploadImages.vue#L7-L8
• [x] Edit the error catcher on the upload POST request: on the if branch change the body to that.uploadError = error.response.data https://github.com/IS-AgroSmart/AgroSmart-Web/blob/c226455c9dcc93948d737b1349666fae23449540/frontend/src/components/UploadImages.vue#L65-L71
• [x] Add the requisite test cases:
  • [x] One to disallow the button if the user attempts to upload over 1000 images
  • [x] One to disallow the button if the user attempts to upload over his remaining quota of images
  • [x] One to show the error message if the API returns "over image quota" error
  • [x] Update the disk quota error test case, since the API needs to return extra data on the HTTP 402 body.