fix: make adding contributors case-space-insensitive

[RonLek]

Issue

Leaderboard was case-sensitive to usernames. Additionally, leading and trailing spaces while adding a contributor would be added as a separate contributor.

Fix

Lowercase and trim new usernames and compare against lowercased cached contributor list.

[RonLek]

@yash-rajpal @Sing-Li could you please review this PR? Thanks.