LDAP fixes (#6533)

* Add iCheck png files to webpack config (inconsistency for css <> png) and blue.png to public folder

* php 7.3 collect() fix (undefined variable)

* Fix travis ci

* Add iCheck png files to webpack config (inconsistency for css <> png) and blue.png to public folder

* php 7.3 collect() fix (undefined variable)

* change LDAP implementation from model to (singleton) service

* Re-apply check for content in ldap_server variable before parsing

* Update LDAP implementation

* Switch iCheck to minimal as referenced in js

* Don't init on load but on first access via init (returns ldap enabled status)

* Re-Enable notifications

* Re-add missing test target php versions

* Only init() once (singleton class, so ldap variable is already set)