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.
* 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
