MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

model/user.go

@@ -90,6 +90,7 @@ type User struct {
 	Timezone               StringMap `json:"timezone"`
 	MfaActive              bool      `json:"mfa_active,omitempty"`
 	MfaSecret              string    `json:"mfa_secret,omitempty"`
+	RemoteId               *string   `json:"remote_id,omitempty"`
 	LastActivityAt         int64     `db:"-" json:"last_activity_at,omitempty"`
 	IsBot                  bool      `db:"-" json:"is_bot,omitempty"`
 	BotDescription         string    `db:"-" json:"bot_description,omitempty"`
@@ -124,6 +125,7 @@ type UserPatch struct {
 	NotifyProps StringMap `json:"notify_props,omitempty"`
 	Locale      *string   `json:"locale"`
 	Timezone    StringMap `json:"timezone"`
+	RemoteId    *string   `json:"remote_id"`
 }
 
 //msgp:ignore UserAuth
@@ -512,6 +514,10 @@ func (u *User) Patch(patch *UserPatch) {
 	if patch.Timezone != nil {
 		u.Timezone = patch.Timezone
 	}
+
+	if patch.RemoteId != nil {
+		u.RemoteId = patch.RemoteId
+	}
 }
 
 // ToJson convert a User to a json string
@@ -734,6 +740,11 @@ func (u *User) GetPreferredTimezone() string {
 	return GetPreferredTimezone(u.Timezone)
 }
 
+// IsRemote returns true if the user belongs to a remote cluster (has RemoteId).
+func (u *User) IsRemote() bool {
+	return u.RemoteId != nil && *u.RemoteId != ""
+}
+
 func (u *User) ToPatch() *UserPatch {
 	return &UserPatch{
 		Username: &u.Username, Password: &u.Password,