Hide actions on tasks until hovered.

![untitled](https://cloud.githubusercontent.com/assets/4495750/4041906/fe108d7c-2cfc-11e4-9dfe-3904d90bf9fc.png)

This PR hides the  push to top, edit, delete, and chart buttons until tasks are hovered on, which gives them a bit of a cleaner aesthetic.
Convo still going on at https://github.com/HabitRPG/habitrpg/issues/3921, this PR will probably be a continued WIP for a bit. (Thanks for tackling this BTW @Gaelan). Anyway, if you want to exclude mobile per https://github.com/HabitRPG/habitrpg/issues/3921#issuecomment-53481129 (if we decide to go that route), here you could do:

```jade
a(ng-class="{'task-action':!env.IS_MOBILE}", ng-click=...
```