Release v0.15.8

- remove pre-systemd default groups for user creation
- update mkinitcpio hooks
- make user-shell configuration available via profile.conf
- support CAL v3.2.1+
- introduce -d flag to set compression level during ISO creation
- support pacman 5.1 series
- restore functionality to define ISO size via profile.conf
- use short url for webseed on torrents

buildiso: use grub on install media

remove plymouth
remove aufs support

fix thus config for sonar

Changelog:

buildiso/check-yaml:

* dropped shared Packages-Desktop file, sonar made it impractical
* switched iso booting to syslinux for mbr boot
* removed plymouth theme setting in profile.conf, now handled by theme package
* synced initcpio miso hooks with upstream arch
* implemented optional persistence for live media (-m switch)
* reorganized the profile's chroot_dir, the resulting iso dir for squashing moved one level up the tree
* improved ext4 or fat img mounting

Profiles:

* new tags:
    *  '>manjaro'
    * '>sonar'
    * '>basic'
    * 'extra'
* any profile now has standardized Packages-Desktop, no shared list merged
* reorganized Packages-Live, Packages-Root to always produce same install, regardless of netinstall or unsquash images
* calamares netinstall yaml files moved into a separate repo [calamares-netgroups](https://github.com/manjaro/calamares-netgroups)
* renamed the two profile.conf netinstall switches
    * configure calamares for netinstall netinstall=true/false(default: netinstall=false)
        * configure netinstall to use chrootcfg instead of unpackfs (default: unpackfs) chrootcfg=true/false **if** netinstall=true