Previously imports would fail if a merge request note included a
suggestion with an
`ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection`
exception.

This was happening because suggestions were listed as a descendant of
merge requests, but this doesn't work because suggestions are directly
associated with notes, not merge requests, and that association is lost.
Rails also disallows creating intializing a has_many association through
a different object.

We fix this by making `suggestions` a child of `notes` within a merge
request. This doesn't fix previously broken exported project exports,
but new exports will work.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65880

- Added Importable to models/list.rb
- Did unless: :importable? on board validation
- Created changelog
- Modified haml to show issue boards are importable
- Added needed spec tests
- Modified project.json to include board information
- Added relevant models to all_models
- Added relevant models to import_export
- Added relevant models to safe_model_attributes

This backports all changes made to an import/export fixture file from EE
to CE.

https://community.atlassian.com/t5/Jira-questions/Is-it-quot-JIRA-quot-or-quot-Jira-quot/qaq-p/681163Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

Ignore some columns

This reverts commit a5378665.

Fixes gitlab-org/gitlab-ce#42595

Since we can regenerate `description_html` from the `description`, we
should not export it. This avoids some complexity when overriding the
description during an import/export where we would need to invalidate
this cached field.

Now we refresh the markdown cache after the import

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.

When importing from a GitLab archive, an admin can carry over the
assignations. Other users can not.

When a regular user is importing an issue with multiple assignees, the
assignee is replaced with the current user, meaning we would try to
insert current user as an assignee multiple times.

By filtering the array before storing it, the import becomes more
robust.

This removes all usage of soft removals except for the "pending delete"
system implemented for projects. This in turn simplifies all the query
plans of the models that used soft removals. Since we don't really use
soft removals for anything useful there's no point in keeping it around.

This _does_ mean that hard removals of issues (which only admins can do
if I'm not mistaken) can influence the "iid" values, but that code is
broken to begin with. More on this (and how to fix it) can be found in
https://gitlab.com/gitlab-org/gitlab-ce/issues/31114.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37447