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.

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)