database: add helpers to fetch all columns

The current solution of fetching columns based on the GraphQL context
has some limits. While probably not the solution for all use-cases, it
sometimes can be desirable to simply fetch all columns from the database
when retrieving objects.

This commit adds two simple functions doing just that. They can be used
when building SQL queries, like

    query := database.SelectAll(new(model.Email))

and

    rows.Scan(database.ScanAll(&email)...)

database: Remove Apply

database: expand field map for composite fields

This allows models to fetch multiple SQL columns to obtain the data
necessary to compute a single composite GraphQL field, while still
avoiding unnecessary fetches if those fields are not queried.

database.Apply: add support for non-pointer fields

Add legacy webhooks worker implementation

Import GQL server interfaces from gql.sr.ht