Fix sender lookup by also using get_pa_string

• src/oomhelp.cpp (get_pa_string): Add debug output. (get_oom_recipients): Remove now redundant debug output.
• src/oomhelp.h (get_pa_string): Declare.
• src/ribbon-callbacks.cpp (do_composer_action, do_reader_action) (attachEncryptedFile): Use get_pa_string to obtain sender.
• NEWS: Update accordingly