* Replace linkRegex with xurls library
Rather than maintaining a complicated regex to match URLs for
autolinking, gitea can use this existing go library that takes care of
the matching with very little code change to gitea itself. After
spending a while trying to find the perfect regex for all cases this library
still works better as it is more flexible than a single regex ever will be.
This will also fix the following issues: #5844#3095#3381
This passes all our current tests and I've added new ones mentioned in
those issues as well.
* Use xurls.StrictMatchingScheme instead of xurls.Strict
This is much faster and we only care about https? links to preserve
existing behavior.
