basic Localstorage implementation

This is an experimental implementation to store the private key encrypted in the browser's localStorage.

I have only tested it locally, without access to an fts server, can we try this on an internal test server?

To improve:
1. check of successful decryption, either by:
    - save a password hash
    - save a hash of the plaintext data
2. integration into the UI
    - don't require a key input in the delegation window if one is already stored
    - more detailed error reporting
3. think about ways to synchronize this between a user's systems