add missing key props

[rosvik]

(Thanks for adding the PR template @sgriff96 😄)

PR Creator Checklist

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• [ ] You've discussed making your changes with a member of the dev team per contributing rules in the README (Not a "entirely new feature")
• Your changes are free of any lint errors
• Your changes are free of any typescript errors
• You've tested your changes

Summary

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Got some debug warnings due to missing key props. This PR adds keys to the cases I could find, to suppress the warnings and maybe prevent future bugs and performance issues. The cases were:

• Trending communities list
• Community search results
• User search results
• App icon selection list
• Theme selection list

Screenshots

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No visual changes

Test Plan

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With debug enabled, verify that none of these cases gives the "Each child in a list should have unique 'key' prop" warning:

• "Search" tab (trending list is visible)
• "Search" tab -> search for a community
• "Search" tab -> search for a user
• "Profile" tab -> settings -> "Appearance" -> "App Icon"
• "Profile" tab -> settings -> "Appearance" -> "Theme"

[sgriff96]

Oh nice this has been annoying me but i've been too lazy to fix it