Validate listing variant option names are case-insensitively unique

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Currently we sanitize listing variant names but don't validate their uniqueness (at least from the cursory look I applied). This bug could manifest in an listing purchase which accidentally applies the wrong Option if the listing option names were to have multiple casings of the same text.

Testing this case would work as follows:

1. Make a listing with a lot of various options called "Name" in different cases.
2. Then purchase it and see if the right option is chosen. The option application behavior is random so you'll have a 1/n chance of not triggering the error. (Where n is the number of Variant options with the name you used.)

This issue represents agreement on the team that our Listing Variant names should be:

• unique in a case-insensitive way
• persisted as they are provided (without removing case-sensitivity)
• and applied in a case-insensitive way

Acceptance Criteria:

• A listing with variant options "Name" and "NAME" would throw a validation error.
• A listing with variant options "Name" and "OtherName" would be persisted as provided.
• A purchase request to apply options "Name" or "NAME" would match the same option.
• Unit tests around the listing option validation proving the above.

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Note: changes in ethereum-master are being applied to tighten up the Variant Option application logic to no longer accidentally choose the wrong option name. This issue is specifically to improve the validation to ensure all names are case-insensitively unique.