Add Open Graph meta tags to <head> for correct preview
[Open Graph](https://ogp.me/)

Main page example:
```html
<meta property="og:title" content="Nim forum">
<meta property="og:type" content="website">
<meta property="og:image" content="..."/>
<meta property="og:url" content="https://forum.nim-lang.org/">
<meta property="og:site_name" content="Nim forum">
```

Thread example:
```html
<meta property="og:title" content="Thread title">
<meta property="og:type" content="article">
<meta property="og:image" content="..."/>
<meta property="og:url" content="Thread url">
<meta property="og:site_name" content="Nim forum">
<meta property="og:description" content="First n symbols from thread content">
```
