Fix signup page prompt text and adding new onsite contacts on small screens

[ColinToft]

Implementation Description

• Fixed some magic numbers for maximum number of rows in onsite contacts
• Hopefully made onsite contacts work in mobile
• Changed the placeholder text to say "Address and city" instead of just address

Steps To Test

1. Try signing up on mobile (should work now)

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have added tests for my changes
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[github-actions]

Visit the preview URL for this PR (updated for commit 6fa3913):

https://feeding-canadian-kids-staging--pr174-signup-page-mobil-y8iy96gp.web.app

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