Merge branch '413-docker-allow-using-core-image-as-base-image-for-app-distribution-images' into 'master'

Resolve "[Docker] Allow using core image as base image for app/distribution images"

Closes #413

See merge request !550

Resolve "style.scss uses deprecated SCSS"

Closes #410

See merge request !551

Resolve "Can't connect to Redis with default configuration"

Closes #414

See merge request !553

Update dependency celery-progress to ^0.1.0

See merge request !546

Resolve "[Docker] Do not run as root"

Closes #409

See merge request !544

Update dependency django-haystack to v3.0

See merge request !548

Update dependency django-colorfield to ^0.4.0

See merge request !547

Run poetry update

See merge request !545

Previously, we relied on either explicit configuration to place the
file in a writable location, or the webserver being able to write to
the source tree.

With a (probably even replicated) setup where nothing runs as root,
and al lstorages are thus read-only except for the media storage, we
need to put the maintenance file there.

Advances #403

Advances #403

Advances #403

[Docker/K8s] Fix typo in S3 storage class name

See merge request !542

Resolve "Support S3 storage for media"

Closes #405

See merge request !534