Add Github action to automatically diff new schema against old schema with updater applied

[wil93]

No description provided.

[wil93]

It doesn't seem to work very well because pg_dump's output is not very stable :(

Probably we should use something like:

• https://github.com/stripe/pg-schema-diff or
• https://github.com/joncrlsn/pgdiff or
• https://github.com/michaelsogos/pg-diff

[prandla]

order of columns is a genuine property of the schema. i'm not sure if it matters when using sqlalchemy but it does matter when doing e.g. select * from a table in the psql console. unfortunately it seems that reordering columns is not something that postgresql explicitly supports. so to get the order to match up, we would need to either always add new fields at the end of a table, or do some pretty horrible hacks in the migration script (dropping and recreating columns, and then fixing up all foreign keys) to get the columns in the right order.

[gollux]

Maintaining consistent order of columns is too hard and removing ENUM values is plain impossible. So if we want an automated check of schema migration scripts, it must take this things into account.

[veluca93]

Please re-open when a more working version is ready