[#71740212 and #71740198] user can attach files to question and answer
user can delete his answer
add nested_attributes methods to question and answer models, specs are greenx

fix Attachment model and migration

add nested_forms gem and configs

add specs for answer deleting
add answer delete

fix answer deleting  method
move dependent destroy form answer to question model

delete answer specs are green

fix file attachments feature
all tests are green

add nested_form gem ability to upload several files to question