Add banner for test systems

[timobrembeck]

Motivation

Sometimes, it can be dangerous to mix up the test and prod systems.

Proposed Solution

• Add new TEST setting independently from DEBUG mode
• Also support reading this setting from environment variables
• When this setting is True, add a test banner at the top of the screen

Alternatives

Additional Context

Just for reference, e.g. PyPI does it the following way:

https://pypi.org/

https://test.pypi.org/

[steffenkleinle]

@timoludwig the text of the banner is not 100% accurate, the content of test cms is not visible in our beta versions. It is only visible in webnext/dev releases of the native apps. Since its probably just for internal use, we might also keep it as it is as long as nobody complains.

[timobrembeck]

@steffenkleinle ah ok, that's unfortunate. The banner should reduce confusion, not add to it. Do you want to open an issue and suggest a different wording?

[steffenkleinle]

@timoludwig done