REQUEST: key/value storage for the app

I've discussed this with @gliechtenstein and I'm not sure if there's another issue like this, but essentially it would be nice if there was a place to store data in the app (e.g. `$storage`).

There are a couple of use-cases:

1. **Credential management.** Storage of credentials for decentralized apps (where a remote server will have a different IP address etc per user, so I can't use `$keys`). This prevents the user from having to add their ip address, port, username, and password every time they open the app. Also allows me to store more than one credential if the user is managing multiple servers.
2. **Performance optimisation.** Some data shared between different views doesn't need a new network call to access it. The data can be passed via `$params`, but this can get cumbersome in complex apps.

Additionally the data stored in `$storage` needs to be accessible to authorised views to prevent malicious views trying to scoop up data from any data in `$storage`.

The issue names the existing $keys and $params concepts but no files or tests. Start by locating their handling and the existing view-authorization path; the scope is complete when storage supports the stated credential and shared-data use cases while restricting access to authorized views.

Written by the indexing model from the issue text.