Localise schedule times

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

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.

Styling for schedule

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

Chrisjrn/session chair (#94)

* Adds session_detail page

* Adds session chairing information to the schedule grid

* Adds session_list.html

* Nicer volunteer text

Conflicts:
pinaxcon/templates/symposion/schedule/_grid.html

Conflict and migration performed by Sachi King <nakato@nakato.io>
Emoji changed, lca2017_tags to lca2018_tags, remove needless dependency
on django-user-account, use request instead like the remainder of the
calls n the template do anyways.

Remove blocktrans from a number of texts as they break {{ templating }}
and we don't translate anyways.

Modified-by: Sachi King <nakato@nakato.io>