Subversion's SSL support badly broke with the subvertpy work done in

2.0. The new code flat-out didn't work with self-signed certificates,

and seemed to have trouble with verification all-together. This was true

of both PySVN and Subvertpy.
In PySVN's case, the SSL trust callback was never set. The base SVNTool

tried to set it on our Client class for PySVN, but it never got set on

the real Client class provided by PySVN.
Subvertpy was missing a bunch of provider classes that were necessary

for reading and writing the certificate verification files in the

.subversion directory.
In both cases, the new accept_certificate() functions were failing to

actually return the generated certificate. They were also not being

careful with the 'failures' key in the resulting certificate. PySVN

prematurely deleted it (causing us to fail when looking up the failures,

if needed), and Subvertpy was leaving it in.
This brings our support for SSL certificates back to a working state.

Performed the following tests with both PySVN and Subvertpy backends:

Set up a self-signed cert for a new SVN server.
Added the repository to Review Board.
Saw the trust prompt, with all the appropriate cert information.
Clicked "I trust this host."
Saw the repository was added.
Ran 'svn info' on the repository, and wasn't prompted about the cert.
Saw the cert file in .subversion.
Re-saved the repository, and wasn't prompted.