Added a setting for asset timestamp - closes #287

allow to manage this setting from the admin area with
default value to "force"
Fix error during fresh installation

With fresh installation, we cannot instantiate Settings.Setting,
as it will automatically attach some behaviors, eg: Trackable that
assumes that Users plugin is already installed.

Workaround: Use AppModel to save data
Log migration errors
Merge branch 'pr-327' into 1.6