feat: use enterprise_host to match github issue urls

[AndersDJohnson]

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Today our GitHub regex makes some assumptions about the URL format for GitHub issues that is not consuming from enterprise_host configuration. This may result in some false positives, such as if I link to an issue on another GitHub instance (such as a reference to a public GitHub issue from an enterprise GitHub PR).

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We could update our GitHub URL finder to look for URLs starting with enterprise_host. That since it is required for non-public GitHub support, so we should be able to count on it. If one is not configured, we can only look for github.com and www.github.com URLs.

Describe alternatives you've considered

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What we have today mostly seems to work too, if the complexity here isn't worth it, or there are edge cases we've not considered.

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n/a