MM-60987: Move message_export code to monorepo (#28674)

* move message_export code to monorepo

* update license headers

* moving over `dedicated filestore` (#1772) from enterprise master

server/enterprise/message_export/membership_map.go

@@ -0,0 +1,58 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.enterprise for license information.
+
+package message_export
+
+type MembershipMapUser struct {
+	userId   string
+	email    string
+	username string
+}
+
+// Provides a clean interface for tracking the users that are present in any number of channels by channel id and user email
+type MembershipMap map[string]map[string]MembershipMapUser
+
+func (m *MembershipMap) init(channelId string) {
+	if *m == nil {
+		*m = make(map[string]map[string]MembershipMapUser)
+	}
+	if (*m)[channelId] == nil {
+		(*m)[channelId] = make(map[string]MembershipMapUser)
+	}
+}
+
+func (m *MembershipMap) AddUserToChannel(channelId string, user MembershipMapUser) {
+	m.init(channelId)
+	if !m.IsUserInChannel(channelId, user.email) {
+		(*m)[channelId][user.email] = user
+	}
+}
+
+func (m *MembershipMap) RemoveUserFromChannel(channelId string, userEmail string) {
+	m.init(channelId)
+	delete((*m)[channelId], userEmail)
+}
+
+func (m *MembershipMap) IsUserInChannel(channelId string, userEmail string) bool {
+	m.init(channelId)
+	_, exists := (*m)[channelId][userEmail]
+	return exists
+}
+
+func (m *MembershipMap) GetUserEmailsInChannel(channelId string) []string {
+	m.init(channelId)
+	users := make([]string, 0, len((*m)[channelId]))
+	for k := range (*m)[channelId] {
+		users = append(users, k)
+	}
+	return users
+}
+
+func (m *MembershipMap) GetUsersInChannel(channelId string) []MembershipMapUser {
+	m.init(channelId)
+	users := make([]MembershipMapUser, 0, len((*m)[channelId]))
+	for _, v := range (*m)[channelId] {
+		users = append(users, v)
+	}
+	return users
+}