Merge branches 'feature/project-export-ui-experimental' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

Merge branches 'feature/project-export-ui-experimental' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

Merge branches 'feature/project-export-ui-experimental' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

Merge branches 'feature/project-export' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-import

Merge branches 'feature/project-export-ui-experimental' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

Merge branches 'feature/project-export-ui-experimental' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

refactored some namespace stuff and fixed project tree restorer spec. also removing controller so that it belongs to the UI MR

Merge branches 'feature/project-export' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-import

Merge branches 'feature/project-export-ui-experimental' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

Merge branch 'feature/project-export' of gitlab.com:gitlab-org/gitlab-ce into feature/project-import

Merge branch 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export-ui-experimental

# Conflicts:
#	app/controllers/import/gitlab_project_controller.rb

Merge branches 'feature/project-export' and 'feature/project-import' of gitlab.com:gitlab-org/gitlab-ce into feature/project-import

Merge branch 'feature/project-export' of gitlab.com:gitlab-org/gitlab-ce into feature/project-import

# Conflicts:
#	lib/gitlab/import_export/command_line_util.rb

Merge branches 'feature/project-export' and 'master' of gitlab.com:gitlab-org/gitlab-ce into feature/project-export

[ci skip]

Support e-mail notifications for comments on project snippets

While working with project snippets recently, I noticed that notifications would not be sent out for comments on notes. This MR fixes this.

Note: I'm not completely sure why `ProjectSnippets#participants` returns an empty array if you don't include the concern that is already in `Snippets` but didn't dig into it any more.

Closes #2334

See merge request !3987