RepositoryForm has supported an idea of subforms for a while, focusing

exclusively on hosting service subforms. In an upcoming change, we'll be

introducing support for SCMTool-provided subforms as well, and to

prepare for that, this change adds a common set of base classes for

subforms that will be used for all repository-related subforms.
The top-most base class, BaseRepositorySubForm, adds the main support

for loading values for fields, setting overridden help text and labels,

defining common attributes for use by subclasses, and adding an

interface method for saving state back out to an object.
This is further subclassed by BaseRepositoryAuthSubForm and

BaseRepositoryInfoSubForm, which form the bases for

HostingServiceAuthForm and HostingServiceForm, respectively. They

don't currently provide any additional functionality beyond

BaseRepositorySubForm, but are there for some future expansion plans

that will take place post-4.0 sometime with the new repository

configuration UI.
The hosting service forms are a bit slimmer now, as they no longer need

to perform some of the operations now covered in the base classes. Along

with this, better documentation has been added.
This does change the requirements for instantiating these forms, but

this shouldn't affect any properly-written forms that safely pass on

arguments in a custom __init__. Care has been taken to not break the

load() or save() function signatures, though the repository

arguments passed to both are pending deprecation.
Unit tests have been enhanced to better check any state loaded from or

saved to a repository by the form.

All unit tests pass.
Configured some repositories and verified that hosting service state

was loading and saving properly.
Tested the hosting service authentication workflow, with and without

two-factor authentication. Tested re-linking with new credentials.