ENH: rewrite core functionality of submission app

1. Remove `submission.models.ZipFile`
    ZIP files are no more staged
2. Rewrite `submission.forms.py` using model forms
    a. `submission.models` now contains help text, labels for form fields
    NOTE: Making keywords bold to show in HTML is now not done.
3. Rewrite views corresponding to create, edit submissions using class based views
    Removed functions, views from views.py:
        a. `get_form()`, `create_or_edit_submission_revision()`
           `get_license_text()`, `new_or_edit_submission()`,
           `edit_submission()`
    New view handlers:
        a. `BaseSubmission()`, `NewSubmission()`, `EditSubmission()`
        Some of the functions are still retained
        NOTE: New view handlers no longer provide previewing functionality from server. Rather it is now handled with JavaScript
4. Create new module `views` with `create.py`, `show.py`
    a. `create.py` contain views to create/edit submission
    b. `show.py` contain views to display
5. Create new module `submission.storage` to handle storing of submissions in filesystem.
6. Add new methods to `filestorage.dvcs_wrapper.py`
    a. `remove()`, `purge()`, `update()`
    NOTE: Needs to turn on `purge` extension in Mercurial (Hg) repository settings in `.hgrc` file on server

Minor Changes:

1. URL Conf for new submission is changed
    from: `r'^new/(?P<item_type>[a-zA-Z]+)/$`
    to  : `r'^(?P<item_type>[a-zA-Z]+)/new/$'`
2. submit button in `submission/new-item.html` is enhanced to disable on submitting the form to server using JavaScript
3. `person.views.profile_page_edit` view's `item` context is renamed to `form` to support default class based views context in `submission/new-item.html`