replace string_insert_hrefs() again (#1103)

My recent change to UrlLinker regexp (commit 9ab0d88) made it detect
single words as URLs. Instead of fixing that replace UrlLinker with the
code from MantisBT as the main reason for introducing UrlLinker (commit
4d7c0c7) was to cover the case of "username:password@" in the URL, which
MantisBT didn't support by that time. The current revision of MantisBT
does that fine and also Unicode and IPv6.