Add Private data

[redransil]

When a user is creating an asset, add a button next to "Create" to create a private asset. The asset itself and template will be public, but the data will be encrypted and only available to someone with the right credentials. Add the same functionality for the API.

See the asset block, type block and data block in the data model here.

Additional functionality:

• When someone logs in, they should see their private assets unencrypted but with a note or icon marking them as encrypted. We should discuss whether to add another field to the asset block to track this.
• Other people can see the private assets of others listed as usual, but not open them (they are encrypted)
• Someone should be able to share a private asset with others using their public key. When the other person it logged in, they can open these assets.