Improve printed schedule

Show each day on a new page when printed.
Hide elements that do not assist with printed copy (current time, etc).

Localise schedule times

Show local times on schedule and presentation details
if user is in a different timezone to the conference.

Styling for schedule

Update to Bootstrap 4 throughout.
Remove unused blocks and elements from past conferences.
Ensure schedule works in print mode.

The 2018ificiation

I think this removes most references to "hobart", "pycon", and "2017"

There are still some references to some images that we don't have a
replacement for.

Remove boxes

Boxes takes content directly from the DB and drops it into the django
templates. This is rather ugly and goes against keeping as much as we
can in static locations. As such, this is being dropped.

September website changes - Pre-registration (#66)

* Factors rendering of external links into its own template. Automagical!

* Adds wagtail templates for keynotes on the front page

* Migrates to Wagtail 1.6

* Migrates content pages to be a streamfield. Flexibility++.

* Fixes editing of ContentPage bodies

* Adds floating images to content pages

* Fixes the layout of floating images

* Adds anchor links to content pages.

* Adds presentation link to the keynote speaker block model

* LCA-ifies the schedule list

* Refactors cms_pages/content_page into a new base template

* cms_pages/content_page now derives from abstract_content_page

* news_index_page now derives from abstract_content_page

* news_page now derives from abstract_content_page.html

* utility_page now uses the content_page base template

* Factors out _right_floating_image.html

* Themes the presentation detail page

* Themes the speaker profile page.

* Themes the schedule list page.

* Minor work on schedule_conference.html

* Themes schedule_detail.html

* Replaces cradle.svg

* Adds a background image to the schedule pages

* Adds libravatar fallback for speaker profile images

* Adds new background images (must update colophon)

* Adds some magic so that we can have slightly different presentation backgrounds for different pages.

* Adds the sponsor block to the bottom of the page.

* Adds sponsor logos to footer.

* All migrations are now in this tree

* Fixes wagtail migrations

* Adjusts presentation_detail to allow for miniconfs (i.e. no target audience)

* Adds unpublishing to presentation detail

* Adds ScheduleHeaderParagraph, which allows us to add some text to the header of schedule pages.

* Adds NamedHeaderParagraph. It allows us to store header paragraphs in the CMS.

* Date formatting in the schedule

* First step for fixing images

* Allows us to automagically load the colophon

* Adds “publish changes” button.

* Can login with email address

Minimum viable changes to get schedule to display (#64)