Fix garbled RSS item descriptions

Previously, item descriptions wrapped stripped Markdown in a CDATA
section. gorilla/feeds escapes that field, so readers saw a literal
"<![CDATA[". Stripping Markdown also left shortcodes and raw HTML behind
as text.

Now, descriptions are plain text from the rendered post, and full posts stay in
<content:encoded>. Tag feed titles also use a real em dash instead of an
escaped "&mdash;".

Fixes #1778