pretix sync: track product and event changes, add tests for this feature

[ichub]

we need to be sure the following tests exist:

• test for updating event name event in pretix getting reflected in pcdpass
• test for updating product name event in pretix getting reflected in pcdpass
• test for product that we're tracking no longer existing causing pretix sync to fail
• test for event that we're tracking no longer existing causing pretix sync to fail

[robknight]

test for product that we're tracking no longer existing causing pretix sync to fail

I'm not sure that this does cause Pretix Sync to fail. We fetch all "items" for each event, and if an item was previously present but now is not, we soft-delete it.

We do have activeItemIDs and superuserItemIDs in the Pretix config, so perhaps we ought to be checking that items with those IDs do exist for the relevant events? Not sure if this should cause sync to fail, or should cause some kind of logging.