Initial team service implementation (#18279)

* initial service implementation

* reflect review comments

app/teams/errors.go

@@ -0,0 +1,20 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package teams
+
+import "errors"
+
+var (
+	AcceptedDomainError = errors.New("the user cannot be added as the domain associated with the account is not permitted")
+	MemberCountError    = errors.New("unable to count the team members")
+	MaxMemberCountError = errors.New("reached to the maximum number of allowed accounts")
+)
+
+type DomainError struct {
+	Domain string
+}
+
+func (DomainError) Error() string {
+	return "restricting team to the domain, it is not allowed by the system config"
+}