use same dsl for actions and actions with dropdown (fix #2625)

## Before
Before this PR the index action DSL is quite different and partly very dirty.

### There are two problems:
#### 1. actions *with dropdown* and custom actions has a different mark up than actions *without dropdown* and custom actions
```ruby
actions dropdown: true do |admin_user|
  item 'Grant Admin', grant_admin_admin_user_path(admin_user)
  # here you need to use `item`
end

actions do |admin_user|
  link_to 'Grant Admin', grant_admin_admin_user_path(admin_user)
  # here you need to use a string retuning method
end
```

#### 2. If you want to show two custom actions (without dropdown), you need to do a very dirty thing:
```ruby
actions do |resource|
  (link_to "Foo", foo_admin_resource_path(resource)) + " " +
  (link_to "Bar", bar_admin_resource_path(resource))
end
```

## After
After this PR both actions types use the same DSL and you don't need to concat strings to display 2 custom actions
```ruby
actions do |resource|
  item "Foo", foo_admin_resource_path(resource)
  item "Bar", bar_admin_resource_path(resource)
end

actions dropdown: true do |resource|
  item "Foo", foo_admin_resource_path(resource)
  item "Bar", bar_admin_resource_path(resource)
end

# You can use the arbre too:
actions dropdown: true do |resource|
  a link_to "Foo", foo_admin_resource_path(resource)
  a link_to "Bar", bar_admin_resource_path(resource)
end
```

NOTE: the old way still works, but should be depreciated

This PR fix #2625