Research changing origins in Safe Apps iframe

[mmv08]

What is this feature about? (1 sentence)

Researching possible improvement for https://github.com/gnosis/safe-react/pull/2188#discussion_r621119174

Why is it needed? What is the value? For whom do we build it?

To make the Safe even more secure

High-level overview of the feature

Possible improvements:

• Before doing a postMessage, prompt the user when the URL isn't the app URL
• pass URL in sdk message
• Apps to define their allowed URLs in the app's manifest.json