Feature: #50780 - Append element browser mount points

See Issue #50780

Description

A new UserTSconfig option is introduced:

options.pageTree.altElementBrowserMountPoints.append

This option allows administrators to add additional mount points in the RTE and the Wizard element browser instead of replacing the configured database mount points of the user when using the existing UserTSconfig option:

options.pageTree.altElementBrowserMountPoints

Usage:

Set these options in User TSconfig:

options.pageTree.altElementBrowserMountPoints = 20,31
options.pageTree.altElementBrowserMountPoints.append = 1

Impact

Mount point overriding is centralized in the BackendUser object and used by element browsers of rtehtmlarea and recordlist for calculating the page tree mount points that are displayed to the user.