Rewrite the new module such that nothing needs to be downloaded

[JonasCir]

Fixes #6

@POPPIN-FUMI We now have a proper new command.
PR features:

1. No downloading of templates (you can remove the old Google storage now)
2. The files we create are now stored as templates. We can easily tweak/improve them over time to our liking
3. Generation of the migration is programmatically done through sea-orm-cli directly
4. Integration testing has been become super convenient b/c of assert_directory_equality
5. I cut down the files and content of the emitted to the bare minimum. I'm not a fan of feature-creep, if we notice that something is missing, we can simply add/improve it.

Please note that generation (which would be the next step after running zapp new) is currently broken. This is due to the fact that a) the code we generate is not compatible with the sea-orm version we are pinning and b) sea-orm is migration based, so we need to switch this (gone file an issue about that later).

[POPPIN-FUMI]

got much cleaner 🎉