Make issue assigned filters more uniform across group and project.

This PR makes the `Assigned to me / created by me / everyone's` issue filter more uniform between project, dashboard and group issues.

The only UI change is that for the dashboard and group, the order changed from:

- `assigned to me`
- `created by me`
- `everyone's`

to:

- `everyone's`
- `assigned to me`
- `created by me`

which matches the order under projects. Those two should have the same order: it makes the UI more uniform.

Note that the default values for those attributes is different between views and has not been changed: for dashboard it is `assigned to me`, for groups depends on signed in or not, and for project issues it is always everyone's.

The greater uniformity also allowed a large factorization. 

![screenshot from 2014-10-02 11 11 14 factor filter state assigned](https://cloud.githubusercontent.com/assets/1429315/4488887/202d0122-4a14-11e4-9719-1a03bf13135a.png)

Implementation notes:

- removed the  `.scope-filter` class which is not used anywhere
- removed the unused `entity` argument to facilitate polymorphism 
Prefer `Object#is_a?` over `Object#kind_of?`.