Replace the CouponCode gem

[ankonym]

As per Elijah's comments below:

Remove the coupon gem and instead generate codes with something like this:

Base32.encode(SecureRandom.random_bytes).downcase[0..7]

or

Base64.encode64(SecureRandom.random_bytes).downcase.gsub(/[0oi1+_/]/,'')[0..7]

The first requires the gem base32, but the latter uses only built-in libraries (require 'base64' and require 'securerandom'). This seems better than depending on the coupon gem, especially since we don't use any of the "features" of the coupon gem.

[...]

Base64.encode64(SecureRandom.random_bytes).downcase.gsub(/[0oil1+_/]/,'')[0..7].scan(/..../).join('-')

=> "myk9-qjdb"

We may want the length configurable in the future, but I think anything longer than 8 characters is unnecessary.