enable domain substitutions in imported urls

test plan:
* create a course with html content, with links pointing to a given
domain (e.g. "http://www.example.com/somewhere")

* in the rails console, manually configure the account settings
as follows, (replacing -COURSEID- with the id of the course):

 a = Course.find(-COURSEID-).root_account
 h1 = {"http://www.example.com" => "https://www.not-example.com"}
 h2 = {:domain_substitution_map => h1}
 a.settings[:default_migration_settings] = h2
 a.save!

* copy the course

* the links should all be translated to the new domain
 (e.g. "https://www.not-example.com")

fixes #CNVS-12325

Change-Id: I0ac2ddf628303ae760995e44c72e2c90a836f966
Reviewed-on: https://gerrit.instructure.com/33472
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>