make it possible to use cloud storage for git lfs and gitea in general

[patcito]

There is already a PR for this here go-gitea#7795

What needs to be done:

• make gitea use an filesystem abstraction for files using this project https://github.com/spf13/afero, see this comment
• make this PR work with it and make sure to rebase it on latest master
• add tests to the gocloud code
• migration tool from file:// to cloud
• make sure file names are reversible

Use min.io server for testing https://docs.min.io/docs/minio-quickstart-guide.html

Use https://github.com/fsouza/fake-gcs-server to test gcp storage.

[cuonglm]

We can close this after #5