Replace linkRegex with xurls library (#6261)

* 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.

vendor/github.com/mvdan/xurls/tlds_pseudo.go

@@ -0,0 +1,24 @@
+// Copyright (c) 2015, Daniel Martí <mvdan@mvdan.cc>
+// See LICENSE for licensing information
+
+package xurls
+
+// PseudoTLDs is a sorted list of some widely used unofficial TLDs.
+//
+// Sources:
+//  * https://en.wikipedia.org/wiki/Pseudo-top-level_domain
+//  * https://en.wikipedia.org/wiki/Category:Pseudo-top-level_domains
+//  * https://tools.ietf.org/html/draft-grothoff-iesg-special-use-p2p-names-00
+//  * https://www.iana.org/assignments/special-use-domain-names/special-use-domain-names.xhtml
+var PseudoTLDs = []string{
+	`bit`,       // Namecoin
+	`example`,   // Example domain
+	`exit`,      // Tor exit node
+	`gnu`,       // GNS by public key
+	`i2p`,       // I2P network
+	`invalid`,   // Invalid domain
+	`local`,     // Local network
+	`localhost`, // Local network
+	`test`,      // Test domain
+	`zkey`,      // GNS domain name
+}