This adds the first step of the new "Connect a service" flow on the

Connected Services admin page. Clicking "Connect a service" now opens a

wizard dialog with a searchable, sectioned list of available hosting

services ("Popular", "Source Hosting", "Issue Tracking"). Choosing a

service loads its connect UI as a server-rendered fragment and submits

the authentication form over AJAX, creating the HostingServiceAccount.
On the backend, this adds a ConnectServiceView that serves the

per-service connect fragment (GET) and processes the auth form (POST),

returning JSON with either a redirect on success or the re-rendered form

with errors. Authorization, two-factor, and certificate errors are

caught and surfaced inline, and unexpected errors are logged.
BaseHostingService now has overridable methods so services can customize

or replace the connect UI.
The wizard view also includes back-navigation and deep-link hooks

(initialConnectURL / data-connect-page) staged for the upcoming

GitHub App connection flow.

Ran unit tests.
Used the wizard to connect new hosting service accounts, exercising

  the success case as well as form errors.