* Build: use golangci-lint as a make command
* Since gometalinter was deprecated in favor of golangci-lint so it was
replaced by it. Responsibilities held by the gometalinter was moved to
golangci-lint
* There was some changes in implementation (that was also mentioned in
the code comment) between the tools, which uncovered couple errors
in the code. Those issues were either solved or disabled by
the inline comments
* Introduce the golangci-lint config, to make their
configuration more manageable
* Build: replace backend-lint.sh script with make
See,
$ gometalinter --vendor --deadline 10m --disable-all --enable=megacheck ./...
pkg/services/sqlstore/user_auth_test.go:28:18⚠️ this result of append is never used, except maybe in other appends (SA4010) (megacheck)
