Enable 3-Tier Server Migration

**Background:**

My company uses a 3 tier deployment strategy, and Wordpress Move has become an integral part of our workflow.  The only thing that slows us down, is that after migrating a database, we have to reset the FTP settings at each step to promote content to the next level.  Our solution is to store multiple FTP configurations in the database so they are always available. This facilitates simple pushing of content both up and down the chain.

**Tiers (and how we use them):**

- Dev : Internal review and integration
- Staging: Client review
- Production: Public

**Changes:**

* Settings pane to includes 3 sets of server configurations
* "URL" added to settings pane, which prefills the "new_domain_name" field during migration.
* Migrate "Begin" button replaced with 3 buttons (one for each server configuration)
* do_migrate accepts the GET argument "server" to specify which configuration to use.