Store contents (list) of archive files in filebase (in the "msg tail")

This will allow fast/easy display of archive contents without actually reading the archive files.

Introduces some new functions:
- list_archive_contents()
- smb_adddfile_withlist()

A new SMB convenience variable ("tail", aliased as "content" for a file).
A new file detail level ("file_detail_content", exposed in JS as FileBase.DETAIL.CONTENTS) which adds a "content" array property to file metadata objects for JS FileBase.get().

Files already added to the new filebases won't have this archive content automatically - I'm looking into that now (likely a new or updated JS script to run).