Add read_channel_content permission (#24018)

* Add read_channel_content permission

* fix tests

* update system console default permissions per role

* add read_channel_content to e2e defaultRolesPermissions

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

server/public/model/migration.go

@@ -40,5 +40,6 @@ const (
 	MigrationKeyAddPlayboosksManageRolesPermissions    = "playbooks_manage_roles"
 	MigrationKeyAddProductsBoardsPermissions           = "products_boards"
 	MigrationKeyAddCustomUserGroupsPermissionRestore   = "custom_groups_permission_restore"
+	MigrationKeyAddReadChannelContentPermissions       = "read_channel_content_permissions"
 	MigrationKeyElasticsearchFixChannelIndex           = "elasticsearch_fix_channel_index_migration"
 )