Finish up off-chain part for English Auction example

[matiwinnetou]

Here is an English Auction in Aiken (https://gist.github.com/matiwinnetou/cae2685b7e88db4e310245f84b5e802c) it is one of Plutus Pioneers middle level complexity exercises. Unfortunately test coverage for it is quite low and off chain part is missing.

This task is about turning this into a proper production grade code with nearly 100% test coverage and bug fixes if there are any necessary.

Plutus Pioneers auction can be seen here:

• https://plutus-pioneer-program.readthedocs.io/en/latest/pioneer/week1.html
• https://www.youtube.com/watch?v=Bj6bqRGT1L0