Add ContextUser to http request context (#18798)

This PR adds a middleware which sets a ContextUser (like GetUserByParams before) in a single place which can be used by other methods. For routes which represent a repo or org the respective middlewares set the field too.

Also fix a bug in modules/context/org.go during refactoring.

integrations/api_user_org_perm_test.go

@@ -133,7 +133,7 @@ func TestUnknowUser(t *testing.T) {
 
 	var apiError api.APIError
 	DecodeJSON(t, resp, &apiError)
-	assert.Equal(t, "GetUserByName", apiError.Message)
+	assert.Equal(t, "user redirect does not exist [name: unknow]", apiError.Message)
 }
 
 func TestUnknowOrganization(t *testing.T) {