Make amount of users in patch configurable.

This PR changes the following:

* Create an API config section for API configurables.
* Add a setting `UserPatchLimit` that controls how many users can be changed in a PATCH request.
* Use this setting in the API to limit the amount of users that can be changed.

services/graph/pkg/config/config.go

@@ -19,6 +19,8 @@ type Config struct {
 
 	HTTP HTTP `yaml:"http"`
 
+	API API `yaml:"api"`
+
 	Reva          *shared.Reva          `yaml:"reva"`
 	TokenManager  *TokenManager         `yaml:"token_manager"`
 	GRPCClientTLS *shared.GRPCClientTLS `yaml:"grpc_client_tls"`
@@ -85,6 +87,11 @@ type Identity struct {
 	LDAP    LDAP   `yaml:"ldap"`
 }
 
+// API represents API configuration parameters.
+type API struct {
+	UserPatchLimit int `yaml:"user_patch_limit" env:"GRAPH_USER_PATCH_LIMIT" desc:"The amount of users allowed to be changed in PATCH requests."`
+}
+
 // Events combines the configuration options for the event bus.
 type Events struct {
 	Endpoint             string `yaml:"endpoint" env:"GRAPH_EVENTS_ENDPOINT" desc:"The address of the event system. The event system is the message queuing service. It is used as message broker for the microservice architecture. Set to a empty string to disable emitting events."`

services/graph/pkg/config/defaults/defaultconfig.go

@@ -30,6 +30,9 @@ func DefaultConfig() *config.Config {
 		Service: config.Service{
 			Name: "graph",
 		},
+		API: config.API{
+			UserPatchLimit: 20,
+		},
 		Reva: shared.DefaultRevaConfig(),
 		Spaces: config.Spaces{
 			WebDavBase:   "https://localhost:9200",