Setting DATABASE_OWNER on a supabase database

### Summary

When using graphile-migrate with supabase, it uses the wrong DATABASE_OWNER.

Supabase has a url that looks like this

```
postgresql://[user].[projectId]:[password]@aws-0-us-west-1.pooler.supabase.com:6543/[db-name]
```

So the `:DATABASE_OWNER` variable is set to `[user].[projectId]` instead of `[user]`

### Steps to reproduce

1. Create a supabase project
2. Create a database, user and password in the supabase url (default is postgres)
3. Put the above connection url into a DATABASE_URL environment variable.
4. Run graphile-migrate migrate

### Expected results

The migration runs.

### Actual results

```
graphile-migrate: Running migration '000001.sql'
error: syntax error at or near "."
```

The . is from the find and replace of [user].[projectId]

### Possible Solution

Give the ability to manually override the :DATABASE_OWNER placeholder.