Add "Where did you hear about us" question to page 2 of form.

I'm going to be developing this feature for a client, but I have reservations. I'm using this space to note/discuss these to welcome input from others who use/develop with this (@ayduns)

Thinking only of my clients, the feature would look like:

• Add config options:
  1. "Label for drop-down question (e.g. How did you hear about us?). Leave blank to disable."
  2. Options (probably a text area with custom format like storedvalue|label one per line, which behind the scenes sets up an option group). Some format like if storedvalue begins other_ then it shows a free text 'other' box maybe.
• Update the front-end app to put this form element in.
• Custom field on Contributions to store the value?

My client needs this - and soon (ofc!) - but I don't much like it:

• Part of the point of InlayPay is to make nice forms. The more we add to the forms the less nice they get.
• Feels like a slippery slope. Is one field ever enough? How long before the expectation grows that we need to allow specifying a Profile? (or in my world, more likely a fluentform)
• Must we then manage our own custom field group for Contributions?
• Does the data go on Contributions or ContributionRecurs? Both? Or a separate activity? (hmm, memory stirring, maybe we're already using somethign like this?)
• Could a (subset) Campaign selection be more appropriate?
• Related: utm_* parameters?