fix: avoid leading double slash

[caugner]

Summary

Problem

1. When getting the locale via useParams(), we may get an invalid (unsanitized) locale.
2. When normalizing multiple leading trailing slashes in our Cloud Function, we don't take encoded slashes into consideration.

Solution

1. Always use useLocale() to ensure we get a valid locale.
2. Remove encoded leading slashes by redirecting.

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How did you test this change?

https://developer.allizom.xyz/%2fexample.com redirects to https://developer.allizom.xyz/example.com.

[LeoMcA]

lgtm

[caugner]

@LeoMcA Maybe we should merge #8661 first, and deploy it there to test before.