Copay Support CSS Style Themes

On this PR:

* Fixes mobile version: (responsive navbar). 
* Fix logo as background css. (png instead of a heavy svg)
* Added a sticky footer for future versions.

Support CSS Themes:

* Added support for css files. Add your new style to: css/tpl-<your-style>.css
* On config.js add: themes: ['<your-style'].

Note: if you have more than one element in the array of styles, it will show in the footer.