Add a new "business type" field to self-serve (browser ads) registration

[lukaslevert]

I think it may be good to start capturing what kind of business is signing up for Ads Manager, so we can better understand and segment the audience/users over time. We'll also be able to cross reference this with referrers and life-time value to see what channels are bringing us the best customers in terms of both dollars spent and quality/size of businesses.

I know we just went through the trouble of removing unnecessary clutter from the intake form, but I think it could help us optimize the quality of different marketing channels. I'm open to putting this in the post-confirmation screen if preferred, but that might miss a lot of the low quality signals we would want to see before we reject them. Thoughts? @tackley @IanKrieger @iambrianfung

Suggested field
What best describes your business?

• Enterprise
• Startup
• Small business
• Agency