FIX: Remove g and m flags from autolink regex (PR #3175)

Fixes issue where a non-link is falsely identified as a link and given the link text of a later link.

More info in thread: https://meta.discourse.org/t/http-gets-parsed-incorrectly-in-posts/24866

The g and m flags together caused the above case. How this regex works, there isn’t any reason to use g as the regex should match at the start of the given text being tested, and for m that is for matching across lines, and links are not valid with multiple lines.

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You’ve signed the CLA, chapel. Thank you! This pull request is ready for review.

Thanks :+1: