Prepares us for upgrade to Rails 5.2

Set ACL of uploaded objects to not-public (fog-aws) or no ACL at all
(fog-google).  Value is ignored by other supported backends (fog-aliyun,
fog-openstack, fog-rackspace)

This allows uploads to GCS with ACL support disabled.

Partially addresses #47424.

For FileUploaders it's possible that a model has many uploads
and if lease key is created only from model id, it causes that
the model's uploads can not be migrated in parallel because the
exclusive lease key would be same for all uploads of the model.

This is tricky: the query was being run in
`ObjectStorage::Extension::RecordsUploads#retrieve_from_store!`, but we can't
just add batch loading there, because the `#upload=` method there would use the
result immediately, making the batch only have one item.

Instead, we can pre-emptively add an item to the batch whenever an avatarable
object is initialized, and then reuse that batch item in
`#retrieve_from_store!`. However, this also has problems:

1. There is a lot of logic in `Avatarable#retrieve_upload_from_batch`.
2. Some of that logic constructs a 'fake' model for the batch key. This should
   be fine, because of ActiveRecord's override of `#==`, but it relies on that
   staying the same.

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

Old records have a null value of file_store column.
This causes the problems with current direct_upload implementation,
as this makes it to choose Store::REMOTE instead of Store::LOCAL.

This change moves the store save when change saving the object.

Introduce `direct_upload` option for `uploads` which is gonna set a default storage to Object Storage and use Unicorn to save data

Fix migrate! method (Minimal fix with ExclusiveLock to prevent race conditions)

Closes #4928 and #4980

See merge request gitlab-org/gitlab-ee!4624

Fix migrate! method (Minimal fix with ExclusiveLock to prevent race conditions)

Closes #4928 and #4980

See merge request gitlab-org/gitlab-ee!4624

Move uploads to object storage

Closes #4163

See merge request gitlab-org/gitlab-ee!3867