ENYO-2282: enhance & fix Ares plugins support

Tested on ChromeCanary/OSX, IE10/Win7.

* ENYO-2282: `ide.json` now defines icons with complete path (HEAD, enyojs/ENYO-2282, ENYO-2282)
* ENYO-2282: IDE now sets CORS headers when not set by services
* ENYO-2282: complete plugins load sequence (notify users)
* ENYO-2282: allow pluggable authentication forms in _Ares > Accounts_
* ENYO-2282: substitute `@PLUGINURL@` in `ide-plugin.json`

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>