Include speaker user name in schedule JSON

When a user has permission to see contact details, expose the speaker's
username in the author object, for integration between systems.

Add code to speaker

Add unique identifier (code) to speakers within schedule JSON API
to allow speakers to be identified throughout the schedule.

Embedded schedule view

Allow the schedule to be embedded in external platforms.
Hide header and footer when embedded for easier consumption.

Update to Django 2.2

Upgrade site and modules to Django 2.2. Remove and replace obsolete
functionality with current equivalents. Update requirements to latest
versions where possible. Remove unused dependencies.

Update licence for schedule JSON

Change licence to CC-BY-SA so YouTube gets appropriate licencing for talks.
This does not correspond to the YouTube Creative Common licence, which is
CC-BY-3.0, which is more permissive than we allow.

Fix presentation detail

Do not retrieve schedule if it does not exist for proposal kind section.
Do not show slot information if slot is not assigned.

Fix schedule day view

Retrieve day from URL on page load and open the schedule for that day.

Add track to schedule JSON

Include the track of the slow in the schedule JSON so the miniconf can be identified.

Wrap input CSV in an TextIOWrapper

Closes #24

Taking one out of PyCon's (US) book

We're lock-step with symposion, and upstream is dead.
Vendor it.