Added markdown editor and fallback font for emoji characters

Welp... it's pretty much what the title says.

Added Symbola Emoji, which is a cropped version of Symbola ([Here's the website](
http://users.teilar.gr/~g1951d/)) so that if the user doesn't have any of the specified fonts that contain emoji installed (Android Emoji, Apple Color Emoji, Segoe UI Emoji, Segoe UI, EmojiSymbols, Symbola), it should fall back to this font which via the magic of CSS (actually it's just [@font-face](
https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face)), allowing any user to see Emoji characters.

The markdown editor adds the ability to easily insert into any textarea that allows it (Edit text submission, Compose comment reply, Compose comment, Edit comment, Compose text submission, Compose private message reply, Compose private message, Edit subverse sidebar text) the markdown tags (Bold, Italic, Link, Quote, Code, Bullet List, Numbered List, Large Paragraph Spacing, Horizontal Rule, Table, Heading 1-6) and Emoji since most desktop computers can only insert those via copy-paste. Finally, [here's a video showing it in action](https://www.youtube.com/watch?v=ro0W00GWPwc).