ENH: ACE editor for new-submissions

ACE editor website: http://ace.c9.io/

Use ACE editor for writing code snippet and description while new-submissions

1. ACE editor syntax highlighter assume plain-text for description, python for snippets by default
DEP: remove submission.views.show_file

`show_file` view:

1. Is not used anywhere in the front-end so far
2. Probably intended to support serving raw files of particular submission
3. Appears to be broken

4. The view searches the wrong path when trying to open images thus raising error. However the uploaded images are stored as media files. Django is not intended to serve them

5. The view also serves submitted code snippet code (stored as files).
5.a) The code served is syntax highlighted using Pygments. But Pygments CSS is not included in response
5.b) The code is served separately (for download) by another view `download_submission`. This makes the purpose of the view confused.
ENH: DEP: Use ACE editor to view snippets

1. Use ACE editor for viewing code snippets in submissions
2. ACE editor highlights syntax in front-end directly using plain-text data

Deprecate:

1. `submission.models.Revision.item_highlighted_code` field is removed
2. `utils.highlight_code` method is removed
3. Pygments module no longer required