Iss1237: Hash keys in Parson's proof problems

[smmercuri]

Keys in a two dimensional Parson's proof_steps array are now hashed in Maxima using Base64 hashing (reversibility is required). Within Question Variables:

proof_steps: [
    ["1", "<proof step one>"],
    ["2", "<proof step two>"],
    ...
    ["n", "<proof step n>"]
];
proof_steps: random_permutation(proof_steps);

Authors of Parson's questions should now use parsons_steps instead of stackjson_stringify. Within Question Text:

[[parsons]]
{# parsons_steps(proof_steps) #}
[[/parsons]]

This fixes the randomisation bug for numerical keys (Issue #1237) and moreover hides author keys from inspection.