Make an easier way to create full schema SQL files

[anoadragon453]

Currently full schema SQL files are made manually from creating Synapse SQLite and Postgres DBs from scratch, and then dumping them into SQL files. This is problematic as it only saves the final schema, but does not keep any of the data that is inserted into the DB.

Some of the delta files insert data in the DB which Synapse then assumes is there, causing bugs like #6311.

We should make sure that the schema and the data of a freshly migrated DB are saved in the delta file.

[anoadragon453]

Fixed in #6394