This adds formal support for using the Repository API to create and edit

repositories backed by a hosting service. Previously, the API really

only worked correctly for plain repositories, as it just wasn't possible

to get the right state into the form.
The POST and PUT handlers now accept several new fields for configuring

the hosting service information (type, URL, account username,

service-specicific fields) for repositories and bug trackers. It does

require an existing HostingServiceAccount to be configured, though.

This takes care to pass the right values to the backend form.
With this, customers should now be able to fully automate repository

manaegment via the API.

Unit tests pass.
Manually tested creating some repositories on GitHub and Bitbucket via

curl. Verified the repositories were successfully created, contained all

the correct state, and could communicate with each service's API.