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.

Improve proposal/review styling

Use correct block for page titles. Fix element styles to be consistent
across site. Use fluid layout for review to enable sidebar.

Tuesday & Wednesday work (#30)

* Fixes an incorrect link in dashboard

* Disables the symposion sponsors app

* Adds redirect links for login/logout; adds THEME_CONTACT_EMAIL value

* Re-adds colophon/copyright message.

* Adds AceMarkdownEditor widget

* Tidies up the generated HTML

* Tidies up form snippet behaviour

* Proposals forms now use the markdown editor

* Monkey patches the markdown editor into the speaker form.

* Adds missing field to proposal details

* Fixes #10 — adds a link to random unreviewed proposals

* Minor tweaks