rename skip_callback->suspend_callbacks

refs CNVS-9522

in rails3 ActiveRecord::Base.skip_callback is a real thing that doesn't
do what this does. because of load order, ours happens, but it would be
nice to leave the other available.

to simplify the switch, both suspend_callback and suspend_callbacks are
kept as renames, but suspend_callback should not be used, preferring
suspend_callbacks. suspend_callback will go away in the near future.

skip_callback is temporarily added back in as an alias to
suspend_callback to allow time for fixing a plugin that uses
skip_callback. it will be removed as soon as that plugin is updated.

Change-Id: Iaefd16dde3b6ce575780cb8f721dc826258eef4e
Reviewed-on: https://gerrit.instructure.com/29808
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>