MantisBT Modern UI

Over the past few months I've been working on building a modern UI for MantisBT. The main drivers for this effort are:  (1) a request from MantisBT core team back in January when I delivered the [mantisbt.org](http://mantisbt.org/) modern website that you see live today,  (2) a loud and clear feedback from MantisBT/[MantisHub](http://www.mantishub.com/) users who publicly and privately spoke against the current UI , (3) a personal desire to contribute to MantisBT open source effort with a lasting positive impact on both end users satisfaction and product adoption.

The changes in this pull request are:
* Integrate with [Bootstrap v3.2](http://getbootstrap.com/) framework as the foundation for all Mantis UI elements.
* Use [Fontawesome](http://fortawesome.github.io/Font-Awesome/) for vector based icons.
* Redesign Layout & Navigation
* Update all pages with the new style, look and feel
* Improved experience on mobile devices

The changes here are just the foundation of the MantisBT Modern UI with functional parity to the existing one across all features (including deprecated ones like news).  Bootstrap also provides good foundation for building a mobile friendly UI.  The existing work just scratches the surface in the mobile space, but we can do much more there.  We can also revisit some of the existing workflows to improve the usability and go beyond the modern looking UI.

A live instance can be found [here](http://www.mantishub.com/modern/index.php) where users can login with user name 'reporter' and password 'demo'.

Please note that this PR disables firefox security headers to address the avatar issue reported here:
[0017491](http://www.mantisbt.org/bugs/view.php?id=17491): Avatars are not showing on Firefox

Here are also some screenshots:

![screenshot 2014-08-14 23 24 42](https://cloud.githubusercontent.com/assets/1354889/3930494/e5b30522-2444-11e4-964e-49418ef27bf0.png)

![screenshot 2014-08-14 23 26 23](https://cloud.githubusercontent.com/assets/1354889/3930504/220f147a-2445-11e4-92e2-bcba415e5dc5.png)

![screenshot 2014-08-14 23 27 33](https://cloud.githubusercontent.com/assets/1354889/3930531/d3691f04-2445-11e4-992b-7bf4f221ac0e.png)

![screenshot 2014-08-14 23 28 15](https://cloud.githubusercontent.com/assets/1354889/3930528/d362571e-2445-11e4-8a84-1d252a4a34d6.png)

![screenshot 2014-08-14 23 28 34](https://cloud.githubusercontent.com/assets/1354889/3930530/d369015e-2445-11e4-8a9c-4d57593560bb.png)

Done.