Allow editing Start URL without loosing the cookies

[RE-Specto]

Currently, the url of the web app, once set, the only way to edit it is via expert settings: start url. Editing it causes a full reset of the web app session and cookies. There are multiple important use cases where there is a need to preserve the current web app and it's cookies and other session artifacts, while temporary or permanently changing the url.

[RE-Specto]

Interestingly, just checked, currectly editing the url back to original is restoring the cookies, so it seems that the issue is that currently native alpha is tying the sandbox to specific app url, rather than to a web app instance.