add and remove operations for fund associations with users and compute sites

Changes include new controllers, a bit of routing, an adjusted view, two model methods and some locales.
Improvement proposition:

```ruby
- fund.user_funds.each do |user_fund|
  = link_to admin_user_fund_path(user_fund),
    method: :delete,
    data: { confirm: t('are_you_sure') },
    class: 'btn btn-default btn-xs has_bottom_tooltip',
    title: t('funds.remove_user.help', user: user_fund.user) do
    = user_fund.user.login
    = icon 'times'
```

As a conclusion we will have something like this:

![fund_users](https://cloud.githubusercontent.com/assets/1265430/5364349/3dedd272-7fe3-11e4-9c9b-5e1e3997ab19.png)