Set tabindex on modal.

This is for #20.

I couldn't `brunch build` locally to test.

```bash
17:10:19 mihnea@light client master ? brunch build
08 Apr 17:10:23 - warn: config.paths.test was removed, use config.paths.watched 0 [ 'config.paths.test was removed, use config.paths.watched' ]
08 Apr 17:10:24 - error: Compiling of 'app/styles/application.styl' failed. stylus:685
   681|             display: none
   682|
   683|         input#filterfield
   684|             width: 100%
 > 685|

Arguments to path.join must be strings
```