indicator for the available localizations of a certain key

#40

Security patch

For issue https://nvd.nist.gov/vuln/detail/CVE-2019-3498

use of Python 3.7

Upgrade to python3.7 (#38)

* Update Dockerfile

* Update runtime.txt

to v 3.7.0

* Update index.html

Bug fixes

Fixed No LocalizedString on superuser account if there is no app set yet.
fixed 500 on app/{app}.locale when there is appkeys
update django and djdt

Update Django to 2.0.5

Update django to 2.0.5

Nullify user reference on delete

for issue #19