Show unpublished conference schedule to staff.

This is an addendum to #123 to allow the `schedule_conference` view to
show unpublished conference schedules to staff members.

Display content_override in slot admin list view.

Fix typo and use `content_override` field (instead of `content`) for a
Slot's `list_display`.

Adds twitter_username to Speaker

This change is based on the PyOhio fork located at: https://github.com/PyCon/pycon/search?utf8=%E2%9C%93&q=twitter_username

Fix queryset used in "Not Reviewed" view.

The `speaker` field on the `Proposal` model corresponds to a `Speaker`,
not a `User`, so the old queryset created a ValueError.

Adds set_quantity as a method on CartController.

Refactors add_to_cart to be in terms of set_quantity

Update to latest build system

Closes #3

Don't show unpublished schedule info to non-staff.

Add checks to schedule_list, schedule_list_csv, and
schedule_presentation_detail views to verify that either schedule is
published or that current user is staff before displaying information.

See c7592bc33e2626d84ec46fe1e7ba8e05c73ef74c.

Make sponsor description default empty string.

Set sponsor listing text to an empty string instead of None. If a
sponsor doesn't have a benefit named "Sponsor Description", this will
prevent the text "None" from displaying in the Django template.

Python3 compatibility

Update the StringIO import in symposion.sponsorship.views

Remove duplicate Slot.save method.

Remove "save" method in Slot model since #107 added another "save"
method. This fixes the automated test failures in Travis CI.

Loosen Django package requirement.

Loosen Django requirement to allow versions greater or equal to 1.8.5,
instead of mandating 1.8.5. This makes it easier to use newer releases
of Django (e.g. the bugfix and security releases 1.8.6 or 1.8.7) with
symposion.

fix pip8 warnings

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

Massively upgrade symposion

* Remove markitup (to be replaced with Ace editor)
* Use DUA decorators
* Removed custom signup bits
* Upgraded dependencies
* Added migrations
* Namespaced template locations
* Removed html5parser/sanitizer (for now) - parsing functionality
should be moved out entirely to a hooks
* Replaced ProposalScoreExpression object with a function that returns
F() expressions

Slot: gen name retrun by __str__() when save()

This name also used for itemize slots on admin screen

Signed-off-by: Hiroshi Miura <miurahr@linux.com>