Include formatting check to the make test (and thus also check) rule (#1366)

* Include formatting check to the `make check` rule

... and give it its own standalone target too (make fmt-check)

Show diff on fmt-check failure
Do not allow running "fmt-check" with incompatible go version
Also simplify the `fmt` rule

* Forbid running "make fmt" with Go version != 1.7 or 1.8

* Ignore bindata.go for spelling and formatting checks

also remove duplicated variable assignment for GOFILES

* Run `make fmt`

models/repo_test.go

@@ -11,8 +11,8 @@ import (
 	"code.gitea.io/gitea/modules/markdown"
 	"code.gitea.io/gitea/modules/setting"
 
-	"github.com/stretchr/testify/assert"
 	"github.com/Unknwon/com"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestRepo(t *testing.T) {