[API] Fix URLs in the 'Link' header for 'GET /projects/:id/repository/contributors' when no value is passed for 'order_by' or 'sort'

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

This could only be possible for users that can create merge requests
within a project.

So they need to be a allowed to create a branch and create a merge request.

Goldiloader (https://github.com/salsify/goldiloader) can eager load
associations automatically. This removes the need for adding "includes"
calls in a variety of different places. This also comes with the added
benefit of not having to eager load data if it's not used.

This overrides values defined in the project JSON with the values
provided in project.import_data.data['override_params'].

These could be passed from the API.

[10.6] Prevent notes on confidential issues from being sent to chat

See merge request gitlab/gitlabhq!2366
# Conflicts:
#	app/helpers/services_helper.rb

This means import data doesn't necessarily have to have an import_url
anymore.

The `ProjectImportData` could just contain the override data in it's
serialized data attribute.

The import data is automatically cleaned up after it is finished by
the state machine.

Features could be listed and added through the api, now also removed.
This was needed in the case of gitlab.com as the number of gates that
were ever used just grows and cleaning up is hard.

- Original work for the web interface here:
 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13713

Closes #28857

'/-/' delimiter is used only in UI, in API we don't use it for
other endpoints. To align search endpoints with the rest of API
endpoints, this patch makes '/-/' optional for existing endpoints (to
keep backward compatibility).

Documentation is updated to prefer paths without '/-/'.

and fixes the project description not being imported