Add survey banner

[ghislaineguerin]

Fixes #90

[seancolsen]

I don't intend to review this PR, but I do have some comments pending on the email thread which I'm still composing. I think it's fine for Pavish to review this PR now, but I'd like to request that we hold off on deploying this change to the live site until we've settled the review of the survey over email.

On the email thread, @ghislaineguerin said:

We can always revisit and make changes

I share this sentiment with other things we publish like our website, documentation, and even Mathesar itself — but not surveys. I think it's fine to make small typo-fixing adjustments to surveys as they're running, but I've experienced situations in the past where larger structural changes to surveys have led to significant complications. Analyzing survey results is much easier if all the submissions are structurally consistent. Also, while I would expect people to visit our marketing site periodically, I would not expect them to open the survey link more than once. With each person, we likely have multiple opportunities to ask them to fill out the survey, but only one chance for them to actually fill it out. So I think it's worth placing a very high priority on up-front effort that will help avoid the need to adjust the survey while it's running, even if that effort delays the publication of the survey.

[pavish]

I agree with Sean, and I'm holding off merging until we have a consensus on the mail thread.

[ghislaineguerin]

@seancolsen I went ahead and used your suggestion. I think the shorter labels might be easier to understand.
@pavish you can now review this.