🚚(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/MANIFEST.in

@@ -1,3 +1,3 @@
 include LICENSE
 include README.md
-recursive-include src/backend/impress *.html *.png *.gif *.css *.ico *.jpg *.jpeg *.po *.mo *.eot *.svg *.ttf *.woff *.woff2
+recursive-include src/backend/meet *.html *.png *.gif *.css *.ico *.jpg *.jpeg *.po *.mo *.eot *.svg *.ttf *.woff *.woff2