Implemented adding an attachment to a question

Users can also remove and add attach a new file in edit question.
Currently, users can only upload one file per question. Some
refactoring needs to be done so that add/edit can use some of the same
code. Also the code can be reused for Answers.
link to the attached file can now be seen on the quesetion view page

When a user delete a question, the attached file is also removed.
Again, currently the design only allows users to attach one pdf
file per question. If more attachments are needed, we will have to
change the design slightly.
implemented attaching files to answers

- modified the FilesForPosts model to have an "alias" attribute
to save the file's original name
- refactored and moved some code to attachment.py to be
shared between questions and answers (and later for comments)
- updated dataformat to return the files attached to Posts
- refacted attachService (in questions controller)
- placed a div tag to display links to questions and answers