Simplify new_post cli

[stickies-v]

A couple of follow ups to #649 before everyone gets used to the new cli:

• make the date parameter required since we're now on a monthly schedule, and the default doesn't really make sense anymore
• use positional arguments to simplify the interface
• add a small warning if the PR has already been covered in a previous post to prevent accidental duplicates

New posts are now to be created with:

./contrib/new_post.py <PR_NUMBER> <HOST_GITHUB_USERNAME> <YYYY-MM-DD>

[jnewbery]

ACK 419bc2db2278c89906131bfad0a57a265c480171

LGTM. No review comments 😊