Small tweaks to the registration process, for a better experience with control_panel and call_command_async

This is dependent on #189.

Two recent changes introduced a bit of confusion in the registration process, but have opened an avenue to a slightly simpler experience.

**Current experience:**
* User opens a new tab/window to register on the central server.
* In a background tab, a distributed server page refreshes to see if registration has happened.
* Once central server registration happens, the user is forwarded to the "zone management" page, where their newly registered device does NOT appear (because syncing hasn't occurred yet). (confusing)
* Eventually, in the hidden tab, the page refreshes, shows a "success" message, and initiates a sync.

**New experience:**
* User clicks on the "register" link, and is taken directly to the central server (no new tab)
* After registration, the user is redirected to the registration page, where a success message appears, and syncing is initiated.
* No extra tabs, no messages to close a tab, no slow background refreshing process.  Just clean and tidy, simple!