Migrate from pgapp to sumo_db

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The database code is getting more complex. We should migrate to a layer like sumo db to manage the db entities. The only issue is that we should update sumo db to use pgapp as the new postgresql driver.

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@igaray do you think you can do this?

[igaray]

Yes, I took a look at this and started. It seems it won't be as straightforward as possible for two reasons:

• there is no easy way of making it a gradual transition.
• HP uses sql in ways that escape the repository model

This is more or less the roadmap I have:

• Add sumo_db and sumo_db_pgsql dependencies.
• Translate the db_user, db_channel, db_reminder, and db_holiday modules to sumo_doc behaviors. Just add the schema definitions, try to match the current table definitions as closely as possible and see whether sumo can at least create and query model entities.
• Create repo modules for the aforementioned entities.
• Create a repo extension for holiday ping to house the complex queries.