Remove gemoji and replace it with emoticon_fontify gem.

## Removal of gemoji

* Apple almost certainly doesn't freely licence their Emoji font so
using it without permission is probably legally a Bad Idea™.
* The Gemoji gem has issues disabled so if there's ever a problem
asking questions or getting it fixed is near impossible.
* Gemoji recomends serving the images from /public which won't work in
a mounted engine.
* Serving the Gemoji images from /assets is possible but slows down
precompilation hugely (over 800 images!)
* Do we really need over 800 emoji and is it worth the 4MB penalty?

## Addition of emoticon_fontify

* The emoticon font is properly licenced, free and open source.
* Hundreds less emojis to remember / bloat an app's size.
* Instead of hard to remember codes like `:stuck_out_tongue_closed_eyes:`
you can simply type an emoticon as you normally would and it is
converted :-)
* Helper method is built into the gem.
* The gem is an engine so the necessary font and stylesheet assets are
automatically included in the asset pipeline (just need requiring the usual way).
* As it's a font the emoticons can easily be styled to match the
forum's text color and size, it will zoom well, and display crisply on
double pixel ratio devices.

## Making it work

I have modified Forem to use the gem. A user of forum only has to do a few simple things to get it working.

First one must require the gem so that the asset pipeline can do its thing.

``` ruby
require 'emoticon_fontify'
```

Finally (yep just 2 steps) one has to add the emoticon stylesheet to their `application.css`

``` css
 *= require "emoticon_fontify/emoticons"
```

I'd love to hear any thoughts or suggestions. And feel free to submit an issue on the [emoticon_fontify](https://github.com/sfcgeorge/emoticon_fontify) page if you find anything broken and I'll look into it.