✨ [FR] Add a setting for custom "empty changelog" markers

[webknjaz]

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The current implementation relies on checking whether the string "No significant changes" is present in the command output: https://github.com/sphinx-contrib/sphinxcontrib-towncrier/blob/e58f24c/src/sphinxcontrib/towncrier/init.py#L78.

That check is hardcoded but, in fact, this output may be different if the user decides to throw in a custom changelog template. That is why we need a setting in conf.py that would allow end-users to also override the checked phrase.