Pull Request for Issue ##24539 and #24723 .

Summary of Changes

Fix directory and file traversal/creation/deletion in com_media for a symlinked "images" folder.
See original bug submission at #24539, PR #24723 by @smehrbrodt and extra analysis by @HLeithner .
Please note that the #24723 changes are included in this PR although not via a merge that shows history (simply copy/paste of the diffed line).

Testing Instructions

Have the /images directory as a symlink that points outside the J installation folder. In my test case it points to ../images
1.Through the back-end mediamanager (/administrator/index.php?option=com_media&folder=), try to navigate to a subdirectory by clicking on the subdirectory icon.
2. Through the back-end mediamanager (/administrator/index.php?option=com_media&folder=), try to upload an image by clicking the "+Upload" button in the toolbar.
3. Through the back-end mediamanager (/administrator/index.php?option=com_media&folder=), try to delete an image by clicking the "X" icon at the top-left of a listed image.
4. Through the back-end mediamanager (/administrator/index.php?option=com_media&folder=), try to create a new directory by clicking the "Create New Folder" button in the toolbar.
5. Through the back-end mediamanager (/administrator/index.php?option=com_media&folder=), try to delete a directory by clicking the "X" icon at the top-right of the folder icon.
6. While in the back-end editing an article with TinyMCE (/administrator/index.php?option=com_content&view=article&layout=edit), drag&drop an image from your OS file manager in the content editing area

Expected result

1. UI directory traversal should display the contents of the selected directory.
2. Image uploads take place
3. Image deletions take place
4. Directory creations take place
5. Directory deletions take place
6. Image upload via the drag&drop takes place and the image is inserted in the content area

Actual result

1. The pageload proceeds without any apparent errors yet it still shows the contents of the root /images directory.
2. The file upload fails. No image upload takes place and a red Error box with the message "Invalid folder provided." appears
3. The file deletion fails. No image deletion takes place and a red Error box with the message "Unable to delete: [uploaded-filename]. File name must only have alphanumeric characters and no spaces." appears
4. The directory creation fails. No directory gets created and a green Info box appears with the message "Unable to create folder. Folder name must only have alphanumeric characters and no spaces.".
5. The directory deletion fails. No directory deletion takes place and a red Error box with the message "Invalid folder provided." appears
6. An upload begins and fails with the error message: "Unable to upload file.: [realpathed-base-media-dir]undefined". No file gets uploaded.

Documentation Changes Required

None