Late night bugfixes

Frontend:

• [Added] strike through support for Markdown
• [Fixed] HTML validation
• [Added] support for OpenGraph

Backend:

• [Added] unique indexes to crossref and postscore
• [Changed] refactored link rendering
• [Fixed] init.php and upgrade.php was unable to be executed
• [Fixed] no more dangling crossref and postscore rows on user/post removal