Release notes detection doesn't work for bitbucket.org

[fthomas]

See https://github.com/typelevel/cats/pull/3811. bitbucket.org returns status code 200 for non-existing files.

[fthomas]

One idea to fix this is to pair the URL we want to show in the PR body with another proxy URL that points to the same resource but returns 404 if the resource is not there. For example, for https://bitbucket.org/asomov/snakeyaml/src/master/CHANGES.rst we could use https://bitbucket.org/asomov/snakeyaml/raw/master/CHANGES.rst as proxy URL. The latter URL returns 200 if the file exists and 404 if it does not so we could use its status as status of the former.