MM-32133 shared channel username collisions (#17347)

Support for handling username collisions between remote clusters. Users belonging to remote clusters have their username changed to include the remote name e.g. wiggin becomes wiggin:mattermost.

@mentions are also modified so the munged username is replaced with the original username when the post is sync'd with the remote the user belongs to.

When adding remote users:
- append the remote name to the username with colon separator
- append the remote name to the email address with colon separator
- store the original username and email address in user props
- when resolving @mentions replace with the stored original username

model/auditconv.go

@@ -673,6 +673,7 @@ func (h auditOutgoingWebhook) IsNil() bool {
 type auditRemoteCluster struct {
 	RemoteId     string
 	RemoteTeamId string
+	Name         string
 	DisplayName  string
 	SiteURL      string
 	CreateAt     int64
@@ -686,6 +687,7 @@ func newRemoteCluster(r *RemoteCluster) auditRemoteCluster {
 	if r != nil {
 		rc.RemoteId = r.RemoteId
 		rc.RemoteTeamId = r.RemoteTeamId
+		rc.Name = r.Name
 		rc.DisplayName = r.DisplayName
 		rc.SiteURL = r.SiteURL
 		rc.CreateAt = r.CreateAt
@@ -698,6 +700,7 @@ func newRemoteCluster(r *RemoteCluster) auditRemoteCluster {
 func (r auditRemoteCluster) MarshalJSONObject(enc *gojay.Encoder) {
 	enc.StringKey("remote_id", r.RemoteId)
 	enc.StringKey("remote_team_id", r.RemoteTeamId)
+	enc.StringKey("name", r.Name)
 	enc.StringKey("display_name", r.DisplayName)
 	enc.StringKey("site_url", r.SiteURL)
 	enc.Int64Key("create_at", r.CreateAt)