🚚(backend) rename Impress to Meet

I have updated all references of "Impress" to "Meet".
Migrations were manually updated and not regenerated. Never-mind,
they all will be squashed before the first release.

I have also searched for reference to "Magnify", and replaced them
by "Meet".

While updating the backend sources, I have also fixed other parts of
the project, namely:
- Compose file
- Github documentation and CI
- Makefile commands

crowdin/config.yml

@@ -17,7 +17,7 @@ preserve_hierarchy: true
 files: [
  {
   source : "/backend/locale/django.pot",
-  dest: "/backend-impress.pot",
+  dest: "/backend-meet.pot",
   translation : "/backend/locale/%locale_with_underscore%/LC_MESSAGES/django.po"
  },
   {