BE - Add visual indicators for content imported by GitHub and Gitea

Following work done for #424454 (closed), we need to make sure similar content imported from GitHub or Gitea is updated with an imported_from value.

For Github:

• events
• issues
• merge requests
• notes
• snippets (?)

For Gitea:

• notes
• issues
• pull requests

DB tables:

design_management_designs
epics            
events                    
issues                   
merge_requests           
notes                    
resource_label_events     
resource_milestone_events
resource_state_events   
snippets       
temp_notes_backup 

https://docs.gitlab.com/ee/user/project/import/github.html#imported-data