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 review pages

Show the same options for reviews on the dashboard and on review screens.
Add title to all pages within the review section.

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.

Switch to LCA2020

Remove LCA2018 and LCA2019 styles and content.
Add base LCA2020 styling and update references to year.

Update jquery version to the full version

datatables uses the fade effect

Restore buttons to review tables

- Switch from dataTable (returns a jquery object) to
DataTable (returns a DataTables API object)
- Update versions of libraries

Doesn't quite work, needs the full version of jquery rather than the
slim version.

Make talk audience and format visible to reviewers

* Audience is visible on review details, but not the list. Perhaps
useful for cases where reviwer wishes to only review for a
particular audience. This change adds audience to the list view.

* Format is not visible anywhere. This changes adds it to list and
detail views for the reviewers.

This change also adds some buttons to control visibility of the new
columns.

Update datatables libraries

This fixes some display issues and adds functionality.

Fix review buttons by removing stale compiled JS; also some styling

Essentially cognate with 0673e35 from the pyconau-2017 fork.

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.

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

Adds “unreviewed by you” to the list of proposal links (#16)