Commit c4579277 introduced an extra administrative security feature to
restrict files access to signed in users only, even  if a user has a valid
& signed download URL. The feature, however, did not take into account
public images & files associated with FAQs and pages such as
landing/thank-you pages.

This commit addresses the shortcoming by adding a reference ID (attachment ID)
to the download/access URL, that can be used to deduce the model/object type
that the file request is associated with. The technique will allow us in the
future to enforce ACL at the file level depending on privacy settings and
the security clearance of the user (agent).

* Show faq count of subcategories on the parent category
* Only show count of public FAQs on the client portal
* Show distinct Help Topics when viewing category on client portal

Introduce ability to create sub-categories to knowledge base categories

Previously, the create method was used to create a new instance of an orm
model (which would later result in an INSERT when persisted); however, some
classes require or utilize varying parameters to their create() method,
which PHP7 considers an error. Methods in subclasses must be defined with a
calling signature compatible with the parent class.

This patch shifts the concept of model creation to the constructor. Now, the
constructor of ORM models is required to be compatible with that of
ModelBase class. Now that most models do not define a constructor, this is
much easier to control, and much more logical.

Also, remove an issue where assignments on a relationship field to an
instance of a super class of the foreign model would raise an error. This
was previously addressed by re-classing the instance in the
ModelInstanceManager::getOrBuild(); however that design would create
multiple instances of the same object in memory, which defeats one of the
primary design concepts of the ORM. This patch addresses the issue by
allowing super-classes of the declared foreign model in relationship
assignments.

Strip tags on text input field to prevent potential XSS exploit.

  * Fix translation of pages
  * Localize client thank-you page on new ticket
  * Fix adding new category
  * Fix crash on staff login when upgrading from < 1.9
  * Don't show self locks on ticket queue pages

This allows for things like related COUNTs and such:

```php
ThreadEntry::objects()->annotate(
    'attachment_count'=>Aggregate::COUNT('attachments')
    )
    ->filter(array('attachments__type__contains'=>'image/'))
    ->filter(array('attachment_count__gt'=>0));
```
This would find all thread entries with at least one image attached.

Move the attachments to the sidebar, add the concept of language-specific
attachments. Add a sidebar to the front page. De-emphasize the "open new
ticket" button. Add a knowledge base search feature to front page and also
add the concept of "featured" articles and categories on the front page.

Multilanguage Support via gettext

- added gettext encapsulations to all texts i thought necessary
- added fallback function for the case that the gettext extension isn't loaded
- added browser language detection
- added gettext to the list of optional extensions in setup
- rewritten some of the texts to use sprintf instead of appending strings
- added german translation file
- removed mark_overdue-confirm from cannedresponses.inc.php

extend multi language support and a fex fixes

- Better detection of translation files
- Added functionality to redirect language codes (see redirecting
language codes)
- Ticket Status can be translated
- The Datepicker can be translated
- Extended functionality of 'testlang.php' to show what language code is
used to translate

Forgotten to apply a patch from RC5 to RC6

- Forgotten to change $var to $vars in line 380 of class.mailfetch.php
- Removed unneeded comment

Added php_gettext as primary translation engine

- Added php_gettext support (thanks to Danilo Segan and Steven
Armstrong)
- php_gettext is now the primary translation engine
- Extended language detection functionality

Add error/misconfiguration checks and fix undefined variables