This introduces new attributes to HostingService to completely hide a

hosting service from configuration or to hide specific supported SCMs.

This is done to keep some level of support enabled in Review Board for

hosting service configuration or their SCMs that may no longer be

supported by the service itself.
If configuring an instance that is using an otherwise-hidden hosting

service or SCM, it will continue to be shown during repository

configuration. This ensures that loading a repository doesn't result in

us showing incorrect information.
To mark a HostingService as hidden, simply set visible = False on

the class.
To mark SCMs on a HostingService as hidden, the visible_scmtools

attribute needs to be set to a list of all the visible SCMTool

IDs/names. If this is not set (None is the default), then any code

that accesses it will need to fall back on supported_scmtools (this is

because we would need a class-level property, which we can't do at this

time).

Unit tests passed.
Tested creating and editing repositories with visible HostingServices.
Tested setting some services and SCMs on a service as hidden and verifying

that they didn't show up by default. Then tested by editing repositories

that were using those and verified that they were showing up and could be

saved.