Anniversary and birthday events have same ID in birthday calendar

[inducer]

(Re-reported from nextcloud/calendar#691)

Steps to reproduce

1. Create a contact with both a birthday and an anniversary
2. Use the "birthdays" calendar with https://github.com/pimutils/vdirsyncer
3. Observe an error like the following:

error: andreas_calendar/contact_birthdays: Storage "andreas_calendar_remote/contact_birthdays" contains multiple items with the same UID or even content. Vdirsyncer will now abort the synchronization of this collection, because the fix for this is not clear; It could be the result of a badly behaving server. You can try running:
error: 
error:     vdirsyncer repair andreas_calendar_remote/contact_birthdays
error: 
error: But make sure to have a backup of your data in some form. The offending hrefs are:
error: 
error: '/nextcloud/remote.php/caldav/calendars/andreas/contact_birthdays/contacts-24ffb9a1c0.vcf.ics'
error: '/nextcloud/remote.php/caldav/calendars/andreas/contact_birthdays/contacts-24ffb9a1c0.vcf-anniversary.ics'

Server configuration

Operating system: Debian Linux "buster" (testing)

Web server: nginx

Database: pgsql 10

PHP version:

PHP 7.0.26-1 (cli) (built: Nov 29 2017 09:48:46) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.0.26-1, Copyright (c) 1999-2017, by Zend Technologies

Server version: 12.0.3

Calendar version: 1.5.6

Updated from an older installed version or fresh install: Updated from owncloud 6, through each major version

Signing status (ownCloud/Nextcloud 9.0 and above): No errors have been found.

List of activated apps:

Enabled:
  - activity: 2.5.2
  - admin_audit: 1.2.0
  - bruteforcesettings: 1.0.3
  - calendar: 1.5.7
  - comments: 1.2.0
  - contacts: 2.0.1
  - dav: 1.3.0
  - federatedfilesharing: 1.2.0
  - federation: 1.2.0
  - files: 1.7.2
  - files_pdfviewer: 1.1.1
  - files_sharing: 1.4.0
  - files_texteditor: 2.4.1
  - files_trashbin: 1.2.0
  - files_versions: 1.5.0
  - files_videoplayer: 1.1.0
  - firstrunwizard: 2.1
  - gallery: 17.0.0
  - logreader: 2.0.0
  - lookup_server_connector: 1.0.0
  - nextcloud_announcements: 1.1
  - notifications: 2.0.0
  - oauth2: 1.0.5
  - password_policy: 1.2.2
  - provisioning_api: 1.2.0
  - serverinfo: 1.2.0
  - sharebymail: 1.2.0
  - survey_client: 1.0.0
  - systemtags: 1.2.0
  - theming: 1.3.0
  - twofactor_backupcodes: 1.1.1
  - updatenotification: 1.2.0
  - workflowengine: 1.2.0
Disabled:
  - encryption
  - files_external
  - user_external
  - user_ldap

Nextcloud configuration:

{
    "system": {
        "instanceid": "51c904fc8c46c",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "datadirectory": "\/var\/lib\/owncloud\/data",
        "dbtype": "pgsql",
        "version": "12.0.3.3",
        "appstoreenabled": true,
        "apps_paths": [
            {
                "path": "\/web\/nextcloud\/nextcloud\/apps",
                "url": "\/apps",
                "writable": false
            },
            {
                "path": "\/web\/nextcloud\/nextcloud\/apps",
                "url": "\/apps2",
                "writable": true
            }
        ],
        "dbname": "owncloud",
        "dbhost": "localhost",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "theme": "",
        "maintenance": false,
        "trusted_domains": [
            "ssl.tiker.net",
            "ssl.cbigman.net"
        ],
        "secret": "***REMOVED SENSITIVE VALUE***",
        "loglevel": 2,
        "trashbin_retention_obligation": "auto",
        "overwrite.cli.url": "\/nextcloud",
        "memcache.local": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "localhost",
            "port": 6379,
            "timeout": 0
        },
        "appstore.experimental.enabled": true,
        "filelocking.enabled": true,
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "htaccess.RewriteBase": "\/nextcloud"
    }
}

[georgehrke]

fix is in #7707