Adding a field to 'event' to record which organization organized a workshop.

We need to track which organization did the admin for a workshop (e.g., SSI organizing in the UK, MSL organizing in North America) so that fees can be divided up.  We do this by having another foreign key from the `event` table back to the `site` table, and adding the partner organizations as sites.
Adding field to record admin fee for workshop.

We now collect an administration fee for most workshops, but the
amount may vary, and we may waive it entirely.  This change adds
a field to record that amount.
Merge pull request #43 from gvwilson/adding-organizer-field-to-event

Adding a field to 'event' to record which organization organized a workshop.