Fix repository interactions for SVN repositories using the SVN protocol with SASL

Summary: also makes the UI more general for this username + password business.

Test Plan:

• configure a phabricator repository from the svn server @asherwin provided which is configured for svn protocol with SASL
• observed phabricator failing without my patch
• upgraded my SVN client to support SASL (protip for mac users - http://www.wandisco.com/subversion/download#osx)
• applied patch to phabricator
• restarted daemons
• noted daemon success - diffusion populating nicely

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Maniphest Tasks: T1260

Differential Revision: https://secure.phabricator.com/D2549