[feature] S3 support (#674)

* feat: vendor minio client

* feat: introduce storage package with s3 support

* feat: serve s3 files directly

this saves a lot of bandwith as the files are fetched from the object
store directly

* fix: use explicit local storage in tests

* feat: integrate s3 storage with the main server

* fix: add s3 config to cli tests

* docs: explicitly set values in example config

also adds license header to the storage package

* fix: use better http status code on s3 redirect

HTTP 302 Found is the best fit, as it signifies that the resource
requested was found but not under its presumed URL

307/TemporaryRedirect would mean that this resource is usually located
here, not in this case

303/SeeOther indicates that the redirection does not link to the
requested resource but to another page

* refactor: use context in storage driver interface

vendor/github.com/minio/minio-go/v7/CONTRIBUTING.md

@@ -0,0 +1,22 @@
+###  Developer Guidelines
+
+``minio-go`` welcomes your contribution. To make the process as seamless as possible, we ask for the following:
+
+* Go ahead and fork the project and make your changes. We encourage pull requests to discuss code changes.
+    - Fork it
+    - Create your feature branch (git checkout -b my-new-feature)
+    - Commit your changes (git commit -am 'Add some feature')
+    - Push to the branch (git push origin my-new-feature)
+    - Create new Pull Request
+
+* When you're ready to create a pull request, be sure to:
+    - Have test cases for the new code. If you have questions about how to do it, please ask in your pull request.
+    - Run `go fmt`
+    - Squash your commits into a single commit. `git rebase -i`. It's okay to force update your pull request.
+    - Make sure `go test -race ./...` and `go build` completes.
+      NOTE: go test runs functional tests and requires you to have a AWS S3 account. Set them as environment variables
+      ``ACCESS_KEY`` and ``SECRET_KEY``. To run shorter version of the tests please use ``go test -short -race ./...``
+
+* Read [Effective Go](https://github.com/golang/go/wiki/CodeReviewComments) article from Golang project
+    - `minio-go` project is strictly conformant with Golang style
+    - if you happen to observe offending code, please feel free to send a pull request