Use standard HTTP library to serve files (#24693)

`http.ServeFile/ServeContent` handles `If-xxx`, `Content-Length`,
`Range` and `Etag` correctly

After this PR, storage files (eg: avatar) could be responded with
correct Content-Length.

modules/context/context_test.go

@@ -5,16 +5,16 @@ package context
 
 import (
 	"net/http"
+	"net/http/httptest"
 	"testing"
 
-	"code.gitea.io/gitea/modules/httplib"
 	"code.gitea.io/gitea/modules/setting"
 
 	"github.com/stretchr/testify/assert"
 )
 
 func TestRemoveSessionCookieHeader(t *testing.T) {
-	w := httplib.NewMockResponseWriter()
+	w := httptest.NewRecorder()
 	w.Header().Add("Set-Cookie", (&http.Cookie{Name: setting.SessionConfig.CookieName, Value: "foo"}).String())
 	w.Header().Add("Set-Cookie", (&http.Cookie{Name: "other", Value: "bar"}).String())
 	assert.Len(t, w.Header().Values("Set-Cookie"), 2)