Fix #12227

This function is about 8 times calles for every single page call, when caching this variable I was able to gain a small performance improvement from 20,512 µs to 630 µs profiled with xhprof

Surely, this is no gigantic gain but if we would do that for every function out there...

Some PHPDocs were incorrect, also used this opportunity to replace some deprecated functions.

adding console command to generate javascript translation files based on existing php translation files
read server side translations from json files

remove translation.php

Adding method getNotificationTypes()
Adding method filterNotificationTypes()
Adding method getDefaultTypes()
Adding method translate() and getTypeIcon()
Adding method getGroupParameter()
Adding method getNavigation()
Adding method getNavigation()
Adding method isFilterValid() and getQueryForFilter()
Adding unit tests for \OC\ActivityManager

* test case added to avoid adding them later

provide a way to detect the language that is being used (e.g. for clientside javascript language selection

If an user is able to create folders in /core/l10n/ he is able to execute arbitrary code. Therefore I've added an `issubdirectory` check and removed all potential dangerous characters from `$lang`.

This patch was automatically generated as part of the following inspection:
https://scrutinizer-ci.com/g/owncloud/core/inspections/cdfecc4e-a37e-4233-8025-f0d7252a8720

Enabled analysis tools:
 - PHP Analyzer
 - JSHint
 - PHP Copy/Paste Detector
 - PHP PDepend

Conflicts:
	lib/private/vcategories.php

strtotime() will return false and the date will be formatted as the 0-timestamp
(e.g. January 1, 1970 00:00) otherwise.