User API key hashing

[loleg]

Instead of encrypting user-defined API keys, we would like to remove the ability to read API keys altogether. Instead only the hashed version of the key should be stored, with the same hashing process being applied every time the API key is checked. This follows a security best practice common to applications like Proxeus.

The UI needs to be updated to remove the "preview" of the API key.

Please comment on possible performance impact of the hashing algorithm.

See apikey.go and issue #1