Add an option to "answer more" for the Flutter UX research

[FMorschel]

When starting to answer the research we get a text that says:

"Please indicate your satisfaction with four different aspects of Flutter. The questions are randomly selected from a larger set of questions."

There could be an option here or at the end of the research that allows users that want to answer more questions to do so.

[FMorschel]

Now I've just stumbled across 3 questions which I could not answer since I don't have experience with neither case. You could benefit from asking if I wanted to answer to other categories.

[FMorschel]

@jayoung-lee sorry for the ping

There are also these issues:

• https://github.com/flutter/uxr/issues/141
• https://github.com/flutter/uxr/issues/140