For a long time now, hosting services have had the ability to provide

custom authentication and repository information forms for the

repository configuration page, allowing them to gather information

needed to authenticate with hosting services, locate organization

details, project IDs, or anything else they needed.
Plain repositories (those not backed by a hosting service) were stuck

with a standard repository form containing Path, Mirror Path, Raw File

URL, Username, and Password fields. SCMTools could show or hide some of

these fields through flags, but otherwise had very little control.
This isn't good enough going forward, as we've finally found a need for

some custom UI for an in-progress SCMTool. Rather than bolt the field

onto the main form and control it through another flag (which we did

with Perforce's Ticket Auth and Git's Raw File URL fields in the past),

it's time to give SCMTools the same level of control that hosting

services have enjoyed.
This change moves all support for those fields into a couple of new

subforms, one for authentication and one for repository details.

SCMTools by default use the standard form fields we've used in the past,

but can provide a custom form by setting SCMTool.auth_form and

SCMTool.repository_form to subclasses of BaseSCMToolAuthForm and

BaseSCMToolRepositoryForm (or the "Standard" subclasses of each).

These can augment or replace the existing fields entirely.
Since this is late in the life of Review Board 3.0.x, we're not going to

deprecate any old functionality. However, the plan is for 4.0 to

deprecate a handful of flags on SCMTool, including field_help_text,

supports_raw_file_urls, and supports_ticket_auth, and require moving

to form subclasses to collect this information.

Unit tests passed.
Tested creating and configuring repositories of various types, using both

plain repositories and hosting services.