Handle co-organized but non-publicized special events on Mobilize

[brotskydotcom]

Occasionally, there are Mobilize events that are co-organized by STV folks but which are not from publicized partners of STV. In this case, the event data will be exported to a separate spreadsheet which doesn't include the other Mobilize events, so importing that spreadsheet will wipe out all the Mobilize events. (This, in turn, will wipe out all the event links for the shifts for those events.) So we need two enhancements:

1. Provide a way of marking an event as one of these special events in the imported spreadsheet, and
2. If the spreadsheet without marking is accidentally imported, find a way of force re-linking all the shifts to their correct events once those have been re-imported.

[brotskydotcom]

So here's how the solution works:

1. To mark an event as special, change its "visibility" field in the spreadsheet to be PERMANENT. This will come in as an "Event Visibility*" of PERMANENT on the Airtable side, and the import containing this event will not delete any events on the Airtable side. Also, the events marked in this way will not be deleted during a regular event import run.
2. To force all the shifts to be re-updated when imported, do a shift import with the environment variable FORCE_SHIFT_UPDATES set to a truthy string value.