🚚(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

src/backend/manage.py

@@ -1,12 +1,12 @@
 #!/usr/bin/env python
 """
-impress's sandbox management script.
+meet's sandbox management script.
 """
 import os
 import sys
 
 if __name__ == "__main__":
-    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "impress.settings")
+    os.environ.setdefault("DJANGO_SETTINGS_MODULE", "meet.settings")
     os.environ.setdefault("DJANGO_CONFIGURATION", "Development")
 
     from configurations.management import execute_from_command_line