Fixes: #536

Fixes: #499

Previously, I had changed the default value to "", to force
users to set the SECRET_KEY value (the app refuses to start
if SECRET_KEY is empty).

The problem with that is, out of the box, with the default
configuration, the tests also don't run and complain about the
empty SECRET_KEY.

So, a compromise: revert back to the default value "---".
At runtime, if SECRET_KEY has the default value, show a  warning
at the top of every page.

Fixes: #467

Fix simple typo in docs, libary -> library

There is a small typo in templates/docs/python.md.

Should read `library` rather than `libary`.

Move terraform-provider-healthchecksio to the "API Wrappers"
category, which is more appropriate than "Tools for Self-Hosting".

Move terraform-provider-healthchecksio to the "API Wrappers"
category, which is more appropriate than "Tools for Self-Hosting".

Fixes: #429

Fixes: #440