This introduces /api/hosting-services/, which provides basic information

on the hosting services supported by Review Board.
The resource does not expose all the information on a HostingService.

Right now, I'm keeping to the basic information, such as the name and

capabilities, until I determine what we're going to want to use for the

new repository configuration UI. This UI may impact how HostingService

itself structures data down the road, so it's not worth baking anything

related to that into the API yet.
This work is loosely based on Olessia's UCOSP work, though many

changes were made to better fit in with other resources, to take

advantage of what WebAPIResource provides, and to make use of the new

LocalDataQuerySet (which greatly reduces the amount of code).

Fetched the list resource and compared the output to the values from

the HostingServices.
Followed links to the item resources. Saw those being fetched as well,

with the correct data.
Tested that things like pagination worked (had to limit the number of

results for this).
Unit tests passed.