Improve sanitizing regexp

[ArthurHoaro]

• less permissive comment trim to avoid stripping a LOT of content
• link descriptions: match a Netscape tag to detect the end instead of any tag

Fixes shaarli/Shaarli#902

TODO:

• [x] unit tests

[ArthurHoaro]

Tested. I'd like to include this change in the next Shaarli release.