Include Donor Note field at checkout that appears on Invoice.

[kimwhite]

Is your feature request related to a problem? Please describe.
When a user makes a donation, they often need to specify what the donation is for, or perhaps in memory of.
Adding a field at check can allow the user to add the reason, show the reason to the site owner, and allow it to appear on the invoice.

An email variable for this reason field. !!donation-reason!!

Describe the solution you'd like
Right now, a field can be added using this gist (it's a bit clunky)
https://gist.github.com/kimwhite/f8226a144fa4f8360863f142f3118cb1#file-my_pmprodon_donation_notes-php

This code will be displayed on the Order Invoice.
https://gist.github.com/kimwhite/09ee8fb06973b8f36cda7cf04ab03372#file-my-add-donation-reason-php

Describe alternatives you've considered
It should be optional by level. It should appear under the donation amount.

Additional context

[andrewlimaza]

I feel like User Fields would get fit most customers needs with a donation reason, we could store it to order meta (it's also okay if it is just stored as user meta for most cases) and custom JS can be written for customers that want to dynamically show this field if the donation amount is entered.

The gists do need work, but definitely a good starting point.

I will leave this issue open for a bit.