R1:65710ee2d24b

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…