Track relationships between moved venues

[rlmead]

We'd like a way to track relationships between venues when they move to new locations.

We could consider doing this by:

• Adding two new fields to the venues table, named something like moved_from and moved_to
• Reworking the functionality of the Replace venue button such that:
  • The new venue is created, with the old venue's ID entered in the moved_from field
  • The new venue's ID is entered in the old venue's moved_to field
  • The old venue is soft-deleted