Add migration reading

This prepares the infrastructure to place migration SQL code in the
source repository with paths formatted as `sql/migrations/%04d.sqlite3`.
The numbers need to be counting up from 1.

They are embedded with Go's embed package and are parsed into a slice of
strings where the migration number minus one is the slice index and the
value is the migrations file as string.

This makes handling of migrations in code as easy as possible.

This commit adds also tests for this functionality.

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