Keep track of the project version in base schema

[wonder-sk]

It would be good to keep Mergin project version metadata information somewhere in the database, then we could:

• detect whether the base schema is not modified (by comparing it to the gpkg at the stored project version)
• ask mergin to give us older version of the project (rather than the last one) to first get to consistent state before doing further pull/push actions

Maybe we could use "comment" functionality in postgres to store the metadata:
https://www.postgresql.org/docs/9.1/sql-comment.html
Probably best to store a simple JSON (e.g. project name + project version) so that we can add more metadata in future if needed.

[wonder-sk]

Fixed in #34 and #36