Implement Salesforce users

[octopusinvitro]

The old site was using an OwnerId field in the Contacts table (supporters table) which is a Salesforce user (SalesforceUser.current_user.Id). In this gem, it should go in the fields returned by DonationFieldsGenerator.

In the old site, vendor/plugins/salesforce_integration/app/models/salesforce_user.rb has a definition of current_user which is defined as:

class SalesforceUser
  def self.current_user
    salesforce_username = ENV['SALESFORCE_USERNAME']
    SalesforceModel::User.find_by_Username(salesforce_username)
  end
end