some plugin fixes

This PR fixes two small things:

1. global assets require for plugins
2. at.who autocompletion styling

### 1. global assets require for plugins

Plugins can include assets to all OpenProject pages. This is handled in `app/assets/javascripts/openproject_plugins.js.erb` using `<%= ... %>`. Unfortunately, this only prints the assets array. `<% .. %>` however does not print the array, but actually requires the plugin assets.

### 2. at.who autocompletion styling

at.who (the issue auto-completion in textareas) highlight the currently matched text in white on a bright yellow background. This is hardly readable, so we print that text in black.

see: 
![selection_127](https://cloud.githubusercontent.com/assets/206108/2610150/eff5601a-bb81-11e3-86e9-fbeaa2c9fd2e.png)

versus
![selection_126](https://cloud.githubusercontent.com/assets/206108/2610151/f5ddb432-bb81-11e3-986a-994a451a9ce4.png)


`