[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 is useful for annotated tags, where the deferenced target is not the same as the tag object. At the moment there is no way to differentiate the two through the tag API.

This change adds a "target" property and leaves the existing "commit" property alone so that existing behaviour is not altered.

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.

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.

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

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

Closes #23460

'/-/' 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

* adds basic discussions API for issues and snippets
* reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets)