Add param discount_source=gift to checkout url

[mircobabini]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This one is useful to detect if we are on a checkout page because of a gift (in order to change design, messages...)

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

[mircobabini]

This one would be useless if we had a lookup table for gifts.

See the proposal request: #22
And see the implementation of the lookup table: https://gist.github.com/mircobabini/c694770e217e483b249be9941eb07ff3

[mircobabini]

Mh, maybe there's another way to do it.

If the code => code_id is for a level which is a gift-level... then it's a gift code.

[dparker1005]

We are going to push forward with the order meta approach proposed in #22. Is it ok if I close this PR?