DP-396: Publish messages in the same transaction as other database operations

[jakzal]

I missed it in my previous PRs.

In this PR, I added an InTransaction extension method on DbContext:

await context.InTransaction(async context =>
{
    // Everything in here will be wrapped in a transaction
});

This extension method can then be used in repositories where we have access to the DbContext:

public async Task SaveAsync(Organisation organisation, Func<Organisation, Task> onSave) =>
    await context.InTransaction(async context =>
    {
            Save(organisation);
            // Assumption here, is whatever `onSave` does, it uses the same `DbContext`.
            await onSave(organisation);
            await context.SaveChangesAsync();
    });

For this purpose, I extended SaveAsync with an additional callback parameter. The first argument is saved, then the callback is called, all in the same transaction:

await organisationRepository.SaveAsync(
    organisation,
    async o => await publisher.Publish(mapper.Map<OrganisationUpdated>(o))
);