perf(getIsCommentDoublePost): optimize extremely slow query

[wescopeland]

Adds an index to optimize the query called in getIsCommentDoublePost().

I'm not sure what has changed, but this query:

$lastComment = Comment::where('user_id', $userID)
    ->orderBy('Submitted', 'desc')
    ->first();

is now taking multiple seconds to run. A simple compound index resolves the issue.

This is the root cause of achievement uploads taking a noticeably long period of time from the toolkit.