Replace SeaweedFS with MinIO

[ScribblerCoder]

I faced alot of issues in SeaweedFS that has to do bucket permissions (anonymous read-only access for users). So I decided to replace it with Minio. Users can still use their own external S3 buckets regardless. This PR updates README.md as well

Minio values override. This creates a default bucket to be used by ctfd with policy set to download. This means ctfd can upload files but competitiors can't, they can only read/download which is what we need.

# Override the minio subchart values
minio:
  # -- Deploys Minio (set to false if you want to use an external S3 bucket)
  enabled: true
  buckets:
      # -- Default bucket to be used by CTFd `download` policy means this bucket is readonly for anonymous access (competitors)
    - name: ctfd-bucket
      policy: download
      purge: false
  # -- Ingress configurations of minio (Used by both CTFd and competitiors)
  ingress:
    enabled: true
    hosts:
      - minio.example.com
    annotations:
      # -- Max Body size `0 -> unlimited` (if you are using another ingress controller then look for the equivalent annotation) 
      nginx.ingress.kubernetes.io/proxy-body-size: "0"
  # -- Minio number of replicas
  replicas: 3
  # -- Minio number of drives per replica/node
  drivesPerNode: 1
  resources:
    requests:
      memory: 2Gi
  # -- Minio PVC size (change according to your needs)
  persistence:
    size: 10Gi

This should resolve #18