Schema and migration fixes

**Resolves #281**

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This pull request introduces the following changes:

- The `vrf` field has been removed from the `StaticInterface` and `DynamicInterface` models.
    - (Rationale: `StaticInterface` doesn't need a `vrf` field because it's only allowed to be at its fixed IP. `DynamicInterface` doesn't need a `vrf` field because it's an interface's membership in a range in a network in a VRF that determines the DHCP class it's put it.)
- Migrations don't assign `Vrf`s to interfaces (since that's now impossible).
- Validation prevents putting a `StaticInterface` in a dynamic `Range` or a `DynamicInterface` in a static `Range`.
- The order of migrations has been adjusted so that interfaces can correctly validate their range.
- All the South migrations on the `soa`, `static_intr`, and `dynamic_intr` apps have been deleted and initial migrations have been re-created, describing the current state of the models.
    - (Rationale: Some of those migrations were broken in that they didn't freeze the ORM. An ORM freeze in each migration is necessary for automatic creation of schema migrations. *FURTHER MISTAKES ARE FORBIDDEN*.)

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***This pull request requires that you drop the database and then re-run South and Maintain migrations.*** Sorry, but it was the only way to do what had to be done.