Add usage disclaimer and username association opt-in for certain user surveys

[leonardthethird]

I think we should do the following:

• Have a very concise statement saying that we may use the anonymized responses for certain purposes e.g., impact assessment, funder reporting, marketing.
• Add an opt-in check-box allowing us to associate with the persons username. i.e., have a public testimonial attributed to @[username]. I think this is a nice touch when we use some of the best ones in visible places....which we should do at some point.

@SylvainChevalier also relaunch the Trust in Metaculus one with optimized language.

More context here.