Existing markdown links results in double-wrapping

[SethCohen]

Issue Description:

Release v1.16.0 introduced an error such as when a Release event payload body already contains a markdown version of a PR, Issue, Compare, etc, the action improperly double wraps the content as it only checks for the url link, resulting in the markdown link version failing.
index.js L50-L78

Expected Result:

Actual Result:

Solution:

Check for existing urls and markdown links before trying to convert.