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

Closes #2334

Fix error when trying to create a wiki page

Closes #15527, #15569, #15623, #15630, #15637, #15653, #15870, #16558, #16875, #16987, #17016, https://github.com/gitlabhq/gitlabhq/issues/10317.

See merge request !3924

Fix "remember me" sign in option

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/4006

Prior to this change, 'remember me' was effectively always on because
the `_gitlab_session` cookie was always made persistent. With this
change it becomes possible again to have `_gitlab_session`be a session
cookie when 'remember me' is unchecked.

See merge request !4004

Signed-off-by: Rémy Coutable <remy@rymai.me>

Remove duplicate entry in the CHANGELOG

[ci skip]

See merge request !4002

Backported minimal safewebhook implementation to GitLab CE

This brings a minimal implementation for gitlab-org/gitlab-ce#13478
backported from EE (gitlab-org/gitlab-ee!334).

Also added UI to configure Secret Token

Fixes #15365.

See merge request !3940