As part of gitlab-org/gitlab-pages#385
we have introduced the use of a custom `.golangci.yml` file with some
custom rules for linting.

This replaces the need of downloading and using `golint`, `gofmt`
`go vet` and `gocyclo` manually. We take advantage of the custom
`golangci-lint` docker image as stated in the [Automatic lintinb]
(https://docs.gitlab.com/ee/development/go_guide/#automatic-linting)
section of the Go standards and style guidelines.

This iteration enables a subset of linters, with the remaining
of them enabled on a separate MR as described in the issue above.

The main changes introduced by this linter include:

- gosec: potential hardcoded credentials
- goconst: DRY by declaring and using constants
- gosimple: reduce statements complexity and improve return statements

Jaime Martinez authored 4 years ago and Vladimir Shushlin committed 4 years ago

Passing secrets via command line is not allowed anymore.
A config file should be used instead. The default filename is
`gitlab-pages-config`. The following command line options will
throw an error and prevent pages from running if set explicitly:

- `-auth-client-id`
- `-auth-client-secret`
- `-auth-secret`

It was ever used. See
gitlab-org/omnibus-gitlab/merge_requests/3444#note_194217889 for more
details.

Done as part of gitlab#28781.

Apply a minimal upgrade to Go modules.
* Fix various tests.
* Drop vendoring.