Merge branch 'develop'

app/apps/api/serializers.py

app/apps/api/views.py

app/apps/core/models.py

app/apps/core/static/js/edit/components/source_panel.js

app/apps/core/static/js/edit/components/trans_modal.js

app/apps/core/static/js/edit/store/part.js

app/apps/core/tasks.py

app/apps/users/admin.py

app/apps/users/forms.py

app/apps/users/migrations/0011_groupowner.py

app/apps/users/migrations/0012_create_group_owners.py

app/apps/users/models.py

app/apps/users/tests.py

app/apps/users/urls.py

app/apps/users/views.py

app/escriptorium/static/css/escriptorium.css

app/escriptorium/static/js/baseline.editor.js

app/escriptorium/templates/base.html

app/escriptorium/templates/core/document_part_edit.html

...escriptorium/templates/users/email/invitation_subject.txt