Review Board 3.0.16 introduced support for custom forms for SCMTools,

storing any saved fields in Repository.extra_data. However, this data

was never passed to check_repository(), meaning that validation could

not happen based on this data for plain SCMTools (though it was passed

to hosting services).
This change adds support for passing custom data to

check_repository(). This requires that the implementation supports

**kwargs (a deprecation warning will be emitted if not) and accepts

all arguments as keyword arguments in any order. This will be required

in Review Board 5. All custom form data will then be passed.
In the process, some of the RepositoryForm code has been cleaned up.

There was a _build_repository_extra_data() method that only worked for

hosting services, and was mostly made redundant when per-SCM subforms

were added. It's now been removed, utilizing the subform data instead.

Unit tests pass on Python 2 and 3.
Tested creating new GitHub repositories, verifying that hosting service

forms still worked correctly.
Tested creating new standard Git repositories, with and without the

updates to check_repository().
Tested creating a new repository for SOS making use of form data in

check_repository().