activitypub: Implement an instance-wide actor

An instance-wide actor is required for outgoing signed requests that are
done on behalf of the instance, rather than on behalf of other actors.
Such things include updating profile information, or fetching public
keys.

Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

models/user/user_system.go

@@ -4,8 +4,10 @@
 package user
 
 import (
+	"net/url"
 	"strings"
 
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/structs"
 )
 
@@ -68,3 +70,28 @@ func NewActionsUser() *User {
 func (u *User) IsActions() bool {
 	return u != nil && u.ID == ActionsUserID
 }
+
+const (
+	APActorUserID   = -3
+	APActorUserName = "actor"
+	APActorEmail    = "noreply@forgejo.org"
+)
+
+func NewAPActorUser() *User {
+	return &User{
+		ID:               APActorUserID,
+		Name:             APActorUserName,
+		LowerName:        APActorUserName,
+		IsActive:         true,
+		Email:            APActorEmail,
+		KeepEmailPrivate: true,
+		LoginName:        APActorUserName,
+		Type:             UserTypeIndividual,
+		Visibility:       structs.VisibleTypePublic,
+	}
+}
+
+func APActorUserAPActorID() string {
+	path, _ := url.JoinPath(setting.AppURL, "/api/v1/activitypub/actor")
+	return path
+}