improve gfm autocomplete

Hi, I'm the project owner of `jquery-atwho-rails` ( [At.js](https://github.com/ichord/At.js) )
I have found that gfm autcomplete in gitlab's demo site was very slow.
It's posting Ajax request every times while At.js catch the username.
I have read all issues related to it, and found it's my bad that this plugin is not good enough.
And it seems no one want to tell me about it. so i gonna to do it on my own.

Here is the improvement:

* autocomplete **issues**
* **preload** all data ( emojis, issues, members) when inputor (textarea, input) **on focus**.
* can search by issues's `title` and `id`
* can search by member's `username` and `name`
* Show popup while preceding by space or is new line. #3786

One more thing, welcome any suggestions. Thanks. :smiley: 

### Commits log

* update jquery-atwho-rails to v0.3.0
* autocomplete issues and preload the data.
* add autocomplete_source action to project for gfm autocomplete
* move init_autocomplete layout from head_panel to project_resource