Fixed rare bug in prev/next post

If tag/user/whatever from last search was deleted after viewing a post,
refreshing the page with that post would yield "Invalid tag/user/whatever"
error.

I changed it so that when retrieving previous/next post for latest search query
throws any errors, saved search query gets resetted to empty one.
Fixed CSS problems with Chrome and Firefox
Refactor to HTML structure

- <script> moved outside <ul>
- Youtube posts pass W3C validation