Bug 1029944 - Switch ModalDialog and AppModalDialog from using strings to using l10nIds

* Replace mozL10n.get(l10nId) calls with node.setAttribute('data-l10n-id', l10nId) in ModalDialog and its use cases.
* Fix jshint errors.
* Update ModalDialog test.
Merge pull request #22387 from begeeben/1029944_switch_modaldialog_and_appmodaldialog

Bug 1029944 - Switch ModalDialog and AppModalDialog from using strings to using l10nIds, r= Zibi Braniecki [:gandalf] 

* Replace mozL10n.get(l10nId) calls with node.setAttribute('data-l10n-id', l10nId) in ModalDialog and its use cases.
* Fix jshint errors.
* Update ModalDialog test.