User interface for session key management?

[btrask]

Session keys are used by client applications to talk to the server. We should probably have some user interface for:

• Creating session keys (including keys with reduced permissions, e.g. read-only)
• Viewing the list of outstanding session keys
• Deleting/revoking session keys

It'd be nice if we also helped users configure their default repository, for example by giving them a copy of the file to put in ~/.config/stronglink/client.json. We can even install it for them if the server is running locally.