Remove `imported` column

The following tables have an imported column which is not being used:

design_management_designs
~~epics~~            
events                    
issues                   
merge_requests           
notes                    
resource_label_events     
resource_milestone_events
resource_state_events   
snippets       

Please note: Ignoring the column on the Epic model caused failures in /ee/spec/models/epic_spec.rb. Debugging these failures proved time-consuming so a new issue has been created to address this.

This column added to these tables with this MR: !149570 (merged) as part of work on #424454 (closed).

After merging, we then decided that imported_from was a better name for the column, which was added to tables with this MR: !151782 (merged).

We now need to remove the imported column, which is a three step/MR process of

1. Ignoring the column in all correlating models (17.1) !152620 (merged)
2. removing the column with a migration/s (17.2) !156540 (merged)
3. removing the ignore directive from the models (17.3) !159599 (merged)