HUE-1298 [impala] Multiple Impalad support

Made the DBMS.get method a factory that reuses instances on a per user per interface basis.
Impala sessions close when connectivity is lost.
Load balancing multiple impalad's with a persistent connection
enables Hue to communicate with the same impalad for as long as Hue is running
and the connection is alive.
[pig] Enforce write permissions on the shared scripts

Display an error message and hide the save button if no write permission.
[oozie] Fix delete button misrepresenting the correct permissions

In Workflow, Coordinator and Bundle editors
[core] Adding autofocus on username field on login

IE10 and other browsers
[impala] Hide system database from the main editor dropdown
HUE-2029 [oozie] Prompt to save a workflow before submission if it was modified

Added modal dialog on unsaved submission