Prevent (or warn) when E2EE is enabled at the same time as SSE

[joshtrichards]

Since they're incompatible currently.

Unfortunately as far as I can tell there's no way to specify in the appinfo/info.xml for this and the encryption app a way to prevent them from being enabled in parallel. In addition, since most interaction occurs client-side, simply returning 4xx codes won't necessarily give the appropriate visibility to any error/warning message when attempting to perform encryption related options.

Some ideas:

• Add a setup check under Admin Overview
• Add a post installation check that causes all E2EE client requested operations to basically be blocked (not friendly but at least keeps things from getting screwed up to start with)
• Add functionality to apps store/app metadata to support specifying apps that aren't compatible with each other
• Add a check in both this app and the encryption app that warns during any UI or CLI settings operations/interactions that the other app was detected and that the user has an invalid deployment/configuration
• ?

[sorbaugh]

Many good ideas here that each have merit on their own and since many people seems to running into this "trap", maybe we can at least start with adding a WARNING text. What do you think @artonge ?

PS: The long term solution will be to make both SSE and E2EE compatible with each other.