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

On the `show` of a project that is part of a fork network. We check if
the user already created a fork of this project in their personal
namespace.

We do this in several places, so caching the result of this query in
the request store prevents us from repeating it.

Prior to this commit running
Namespace#force_share_with_group_lock_on_descendants would result in
updating _all_ namespaces in the namespaces table, not just the
descendants. This is the result of ActiveRecord::Relation#update_all not
taking into account the CTE. To work around this we use the CTE query as
a sub-query instead of directly calling #update_all.

To prevent this from happening the relations returned by
Gitlab::GroupHierarchy are now marked as read-only, resulting in an
error being raised when methods such as #update_all are used.

Fortunately on GitLab.com our statement timeouts appear to have
prevented this query from actually doing any damage other than causing
a very large amount of dead tuples.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37916

…when needed

In this instance its subgroups, and given we can't deploy it, we
shouldn't allow it to be shown.

Fixes gitlab-org/gitlab-ce#34864

In EE, we make use of `namespace#public?` in projects. When the project is in a
personal namespace, this breaks as the `public?` helper isn't present.

This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

Group#destroy would actually hard-delete all associated projects even
though the acts_as_paranoia gem is used, preventing Projects::DestroyService
from doing any work.

We first noticed this while trying to log all projects deletion to the Geo
log.

This hides/disables some UI elements and API parameters related to
nested groups when MySQL is used, since nested groups are not supported
for MySQL.