Move reference to htmlsingle to match standard Spring reference layout
SEC-2378: Fix CSRF MultipartFilter doc typo
Remove repository release checks since init script adds automatically
SEC-1932 Added PBKDF2PasswordEncoder.

 - Also moved some logic into a new class, AbstractPasswordEncoder. Both PBKDF2PasswordEncoder and the now-simplified StandardPasswordEncoder extend AbstractPasswordEncoder.
 - Added tests for PBKDF2PasswordEncoder .