[Feature Request] Sort tasks with subtasks by due date of most urgent subtask

Steps to reproduce

1. Open some tasks and some subtasks
2. Sort by due date
3. Find Tasks without subtasks sorted by due date, after them tasks with subtasks in alphabetical order of main task name

Expected behaviour

Tasks should (optionally) be sorted by which task has the most shortly due subtask
I think this should be opt- in, or simply a different sorting option additionally to what there is already.

Actual behaviour

See above

Server configuration detail

Operating system: Linux 4.19.118-v7+ #1311 SMP Mon Apr 27 14:21:24 BST 2020 armv7l

Webserver: Apache/2.4.38 (Raspbian) (apache2handler)

Database: mysql 10.3.23

PHP version:

7.3.19-1~deb10u1
Modules loaded: Core, date, libxml, openssl, pcre, zlib, filter, hash, Reflection, SPL, session, sodium, standard, apache2handler, mysqlnd, PDO, xml, apcu, bz2, calendar, ctype, curl, dom, mbstring, fileinfo, ftp, gd, geoip, gettext, gmp, iconv, igbinary, imagick, imap, intl, json, ldap, exif, msgpack, mysqli, pdo_mysql, apc, posix, readline, redis, shmop, SimpleXML, smbclient, sockets, ssh2, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xmlreader, xmlwriter, xsl, zip, Phar, memcached, libsmbclient, Zend OPcache

Nextcloud version: 20.0.0 - 20.0.0.9

Updated from an older Nextcloud/ownCloud or fresh install: fresh

Where did you install Nextcloud from: unknown

Enabled:
 - accessibility: 1.6.0
 - activity: 2.13.1
 - bruteforcesettings: 2.0.1
 - cloud_federation_api: 1.3.0
 - comments: 1.10.0
 - contactsinteraction: 1.1.0
 - dashboard: 7.0.0
 - dav: 1.16.0
 - federatedfilesharing: 1.10.1
 - federation: 1.10.1
 - files: 1.15.0
 - files_mindmap: 0.0.22
 - files_pdfviewer: 2.0.1
 - files_rightclick: 0.17.0
 - files_sharing: 1.12.0
 - files_trashbin: 1.10.1
 - files_versions: 1.13.0
 - files_videoplayer: 1.9.0
 - firstrunwizard: 2.9.0
 - issuetemplate: 0.7.0
 - logreader: 2.5.0
 - lookup_server_connector: 1.8.0
 - nextcloud_announcements: 1.9.0
 - notifications: 2.8.0
 - oauth2: 1.8.0
 - password_policy: 1.10.1
 - photos: 1.2.0
 - privacy: 1.4.0
 - provisioning_api: 1.10.0
 - recommendations: 0.8.0
 - serverinfo: 1.10.0
 - settings: 1.2.0
 - sharebymail: 1.10.0
 - support: 1.3.0
 - survey_client: 1.8.0
 - systemtags: 1.10.0
 - tasks: 0.13.3
 - text: 3.1.0
 - theming: 1.11.0
 - twofactor_backupcodes: 1.9.0
 - updatenotification: 1.10.0
 - user_status: 1.0.0
 - viewer: 1.4.0
 - weather_status: 1.0.0
 - workflowengine: 2.2.0
Disabled:
 - admin_audit
 - encryption
 - files_external
 - user_ldap

{
    "instanceid": "***REMOVED SENSITIVE VALUE***",
    "passwordsalt": "***REMOVED SENSITIVE VALUE***",
    "secret": "***REMOVED SENSITIVE VALUE***",
    "trusted_domains": [
        "arnodell.hopto.org"
    ],
    "datadirectory": "***REMOVED SENSITIVE VALUE***",
    "dbtype": "mysql",
    "version": "20.0.0.9",
    "overwrite.cli.url": "https:\/\/arnodell.hopto.org\/nextcloud",
    "dbname": "***REMOVED SENSITIVE VALUE***",
    "dbhost": "***REMOVED SENSITIVE VALUE***",
    "dbport": "",
    "dbtableprefix": "oc_",
    "mysql.utf8mb4": true,
    "dbuser": "***REMOVED SENSITIVE VALUE***",
    "dbpassword": "***REMOVED SENSITIVE VALUE***",
    "installed": true,
    "app_install_overwrite": [
        "mindmap_app",
        "files_mindmap"
    ]
}

Are you using external storage, if yes which one: no

Are you using encryption:

Are you using an external user-backend, if yes which one: no

Client configuration

Browser: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:81.0) Gecko/20100101 Firefox/81.0

Operating system: ubuntu

Logs

n/a

n/a

No source files or tests are named in the issue. Start by locating the task-list sorting entry point and the code that represents parent tasks and subtasks, then reproduce the current due-date ordering with nested tasks. Done means an additional, clearly defined sorting option orders parent tasks by their most urgent subtask due date without changing existing sorting.