Save versions of the schema

[cpina]

Currently when the schema is saved it overwrites the previous schema. It might be useful to keep track of versions of the schema: for accountability and also to be able to revert mistakes.

Besides saving the different versions it should be possible to list the changes with the timestamp and the user that made the change. It could also be useful to have a "diff" between changes.

Implementation details: this could be done on saving the schema: add a version field, each save goes to a new version and it should also save the user that saved it. Or a package like https://django-simple-history.readthedocs.io/en/latest/ could be used.

This is related in a way to #5 ("Handle conflicts", as this versioning at least it would avoid overwriting).