Remove django-dbbackup for now.

See merge request BiscuIT/BiscuIT-ng!77

https://github.com/django-dbbackup/django-dbbackup/issues/314

Update dependency flake8-rst-docstrings to ^0.0.13

See merge request BiscuIT/BiscuIT-ng!76

This reverts merge request !61

Resolve "Migrate to Django 3.0"

Closes #75

See merge request BiscuIT/BiscuIT-ng!61

Merge branch '75-migrate-to-django-3-0' of edugit.org:BiscuIT/BiscuIT-ng into 75-migrate-to-django-3-0

Update dependency django-dbsettings to v1

See merge request BiscuIT/BiscuIT-ng!75

Use cp instead of ln.

See merge request BiscuIT/BiscuIT-ng!74

Resolve "Use icons instead of plain text"

Closes #80

See merge request BiscuIT/BiscuIT-ng!73

Resolve "Migrate to material-design-icons"

Closes #116

See merge request BiscuIT/BiscuIT-ng!67

This reverts commit ce766c73.

 * Update django-two-factor-auth to current release
 * Allow phunnumber-field 3.x or 4.x while django-two-factor-auth
   still crunch https://github.com/Bouke/django-two-factor-auth/pull/326
 * Run poetry update
   * Update django-image-cropping and easy_thumbnails for DJango 3.0 support
     (cf. !61 and #75)