Django 2 Upgrade (part 4). Upgrade to Django 2.2 LTS

[noliveleger]

This PR is the final step of 4 PRs for Django 2 upgrade.
It's related to:

• PR Django 2 Upgrade (part 1): Drop Python2 support __future__ imports removal. #2503 (Drop Python 2 support, bulk removal __future__ imports)
• PR Django 2 Upgrade (part 2): Drop Python 2 support #2504 (Drop Python 2 support, the rest of it)
• PR Django 2 Upgrade (part 3): Force explicitly "on_delete" for foreign keys. #2507 (Django Upgrade, add explicit on_delete on models and migrations)

The reviews should be done in the list order. The comparison should rebase on python3-upgrade  or (two-databases if python3-upgrade is already merged) after each PR review.

Closes #2489, closes #2490

TODO: Condense action items into separate list (or list first)

Notes:

• kpi container must be rebuild before starting it. Lots of pip dependencies are upgraded and the code is not retro-compatible with previous versions.

• It looks like the entire Whoosh search index must be removed; otherwise many unit tests fail. Deleting everything inside whoosh_index does the trick; but I (@jnm) need to see if ./manage.py rebuild_index is enough.

• If S3 storage was already configured, KPI_DEFAULT_FILE_STORAGE must be updated for boto3, i.e. KPI_DEFAULT_FILE_STORAGE=storages.backends.s3boto3.S3Boto3Storage

• django-oauth-toolkit in python3-upgrade is version 1.1.0. This version contains a bug

Critical: Django OAuth Toolkit 1.1.0 contained a migration that would revoke all existing RefreshTokens (0006_auto_20171214_2232). This release corrects the migration. If you have already ran it in production, please see the following issue for more details: jazzband/django-oauth-toolkit#589

• django-cors-headers has been updated to latest version. Be aware, now the origin needs to contain the scheme. Entries will need to be updated on production servers.
  https://github.com/adamchainz/django-cors-headers/blob/master/HISTORY.rst#300-2019-05-10

• django-reversion can't be updated to latest version. Latest version removed squashed migrations and this makes updates from the current version we use fail.

• django-markitup is not compatible with Django 2.x. Had to use a fork of it. https://github.com/st8st8/django-markitup

• django-cache-request is not compatible with Django 2.1+. Had to fork it https://github.com/kobotoolbox/django-request-cache

• jsonbfield pip dependency has been removed in favor of Django PostgreSQL native JSONField.

• Django 2 introduced a new default model permissions: view_, which conflicts with our permissions view_asset and view_collection. Respective models have been modified to keep old behaviour.