[MM-59361] custom groups & read only channels (#28892)

* added group mention, read only view and post event tracking

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/store.go

@@ -310,6 +310,8 @@ type ChannelStore interface {
 	SetShared(channelId string, shared bool) error
 	// GetTeamForChannel returns the team for a given channelID.
 	GetTeamForChannel(channelID string) (*model.Team, error)
+	IsReadOnlyChannel(channelID string) (bool, error)
+	IsChannelReadOnlyScheme(schemeID string) (bool, error)
 }
 
 type ChannelMemberHistoryStore interface {