FIX site.js -- allow correct tags in Markdown

[ruv]

The marked module (which converts Markdown into HTML) does not escape <, if it looks like the start of an HTML tag. So, a wrong tag soup can appear in the result.

In the same time, correct HTML tags were used sometimes, e.g. <ins> ... </ins>.

To distinguish correct tags from other use of <, the new regexp does lookahead assertion for the closing tag.

If < and > are used not for a correct tag, they are just escaped via < and >, to avoid an incorrect tag soup in the result.

[GeraldWodni]

I really don't want to allow general HTML. I explictly allowed <ins> and <del>, and checking some of your contributions, I also allowed <u> and <i>