Minimum should be made more robust

[jkroll-cf]

Currently, the UI will not specify a minimum although it shows what appears to be a default. This leads to situations where users create ciphertexts which can't be decrypted (because they require a minimum of 0 owners and don't get any actual encryptions). This should be handled better from a UX perspective:

• The UI & backend should only allow valid values of "Minimum" and should require a predicate when Minimum == 0.
• The UI should offer a dropdown that clearly explains how to use this, to steer people towards not falling into this trap.
  I've seen this happen several (at least 3) separate times, and it's an easy fix.

[jkroll-cf]

Additionally, if Minimum > 0 and a predicate is set, the request should be denied in the UI as it isn't clear which policy to apply to the ciphertext.