Move media field to bs modal

Select image field (field type media) is using mootools modal.
Without affecting in anyway the logic of the field we can use jQuery and bootstrap.

This needs some visual fixes as the rendered modal needs some css to get to a more appropriate size.
Feel free to submit some ideas or code!

B/C
Should be 100% compatible

Test:
Apply the patch 
Go to administrator -> global configuration
and check the functionality of Offline image.
Everything should be ok ~~(except the size of the modal)~~