Populate hosting services on initialization.

Hosting services were only populated when something first made use of
them. This meant that, until that happened, any hosting service URLs
weren't actually registered, and would return a 404.

We now populate when initializing. This incurs a small startup penalty,
but one that would be soon hit anyway, and is necessary for hosting
service URLs to be reliable.

This has been broken since hosting service URLs were introduced in 2.0.

Reviewed at https://reviews.reviewboard.org/r/6319/